The Wave Theory of Slash
The Wave Theory of Slash, often referred to simply as "Wave Theory", is the idea that there are clusters of tropes and styles in slash that a given fandom (or a given fan) progresses through in the same way that feminism is said to have progressed through various "waves".
The Wave Theory of Slash Fandom was put forth by Lezlie Shell on the Virgule-L mailing list in 1993. It sparked a lot of interest as it classified a fandom's fanfiction according to 4 sets of characteristics: explicitness of a sex scene, relationship of the story to canon, sexuality focus within a sex scene, and writer's perceived relationship to the fandom. Because the list was a private mailing list, the information could not be directly passed around to other fans, but had to be alluded to and discussed without accessing the original text; this led to fannish drift.
The theory was revised in 1996 to better reflect the way people were coming into fandom via the Internet. Some clarification was made that fans could drop into the various phases at any point, rather than always starting with phase one.
In 2007, the theory was revised again to shift the focus away from the stories per se and onto the writers and readers, suggesting that the same person might want to read or write different waves in different fandoms, depending on how involved they are in a given fandom (i.e., they may be fourth-wave in their primary fandom, but first- or second-wave in an unfamiliar fandom).
The Wave Theory was one of the first "big hits" of fannish meta fandom, and it has elicited conversations and analysis from the time it was first posted through 2008. 
The Four Phases or Waves (from the original posting)
Phase One: Character-based stories with slash
- 1. The relationship between the characters is the point of the story. Slash is a means to intensify that relationship.
- 2. These stories are almost exclusively set in the "real" broadcast universe as the writers' love of the show/characters as presented is what got them into fandom.
- 3. The writer invests a great deal of time making characters presented as heterosexual having sex with each other "believable". In these stories this relationship is not "homosexual" in the political or social sense. The sex acts are between two people of the same sex, but are not "realistic" in relation to the lives of homosexual men.
- 4. The writers are in fandom (in contact with other fans) and already writing non/slash stories. They view slash as the end of a progression. Would have no trouble classifying a sexless story as slash.
Phase Two: Character-based slash
- 1. Stories about the characters involved in a slash relationship. The slash characterizations are still tied to the aired ones, but the writers do more extrapolation without looking for "proof" in the aired episodes. Certain aspects of the first-wave characterizations are accepted on equal footing as aired source material.
- 2. The majority of the stories are still in the "real" world, but it is a broader world. The few a/u stories are the "real" characters in another time. The reader has no trouble recognizing "aired" characters in these stories.
- 3. The sex in these stories is more realistic in that the writers have probably read "The Joy of Gay Sex", but the sex is still female-oriented.
- 4. Second wave writers are already a part of fandom and are readers of non/slash fan lit, but there is no doubt that reading slash gave them the impetus to write.
Phase Three: Slashing the characters
- 1. The slash relationship is central to the story. Without it, there would be no story. But, let me hasten to add, there IS a story complete with plot.
- 2. No emphasis on trying to convince the readers that these characters are having sex. The characterizations are based on 1st and 2nd wave stories as much if not more than the episodes.
- 3. Sex is more realistic in regards to actual homosexual practices. In these stories, one or both of the characters has experience with the same sex (other than the first-wave Bodie in Africa type of experience).
- 4. The writers were drawn into fandom by the slash. To them, there is no point in a sexless slash story.
- 5. Alternate Universe stories come into their own. The A/U is used to remove the characters from the strictures of the "real" world, or, to put it bluntly, to let the characters be out of character.
Phase Four: Multimedia slash
- 1.Slash goes multi-media. It is commonly accepted that the only admission requirement for a male TV character to be slashed is a penis. The notion that there was something "special" about K&S or B&D, etc. that made them slashable is viewed with tolerant amusement by the 4th wavers.
- 2. The characterizations in multimedia are, for the most part, composite slash characterizations built from fan fiction in other fandom. It takes a VERY VERY good writer to do character-based slash for a show that has a limited audience because the readers buy-in is limited
- 3. Fourth wave sex, particularly for shows set in present-day America, is more sophisticated. Some stories have one or both characters being bi or homosexual, as opposed to just having some same-sex experience.
- 4. While the writer will be drawn into fandom by the virtue of writing, the readers don't get drawn into the fandom.
Responses to the theoryIn 2008, Sandy Herrold reposted one of her repsonses to Lezlie's Wave theory:
"This was posted a few weeks after The Wave Theory of Slash, partially as a response to it, back in 1993:
"Threshold Fandom" theory:
One, it matters 'how' people are brought in, as much as when; i.e., What if the first ten slash stories you read, mark you for your fannish life?
- One possibility is you dislike *all* of them. Unless your entire social group become slashfen all at once, you'll probably never try slash again.
- Another is, there is more than one fandom represented in that stack of 10, and you like most/all of them. You never realize that many of the people around you are faithful to some fandom, and from the beginning, you think of yourself as media (or slash) fan, instead of a B7 or Pros fan. This didn't happen much back in the '70s/early eighties, but it certainly could have happened by the mid-eighties.
- Lots of other possibilities--if whatever you are shown first marks you, then a first waver bringing in a friend today will show them their fav 1st wave stories, and make that new fan a first waver, too; i.e., someone who thinks of a firstwave story as "the real slash". A second-waver will give a new fan 2nd-wave stories, and that new fan will think of second-wave slash as the way that slash should be written, etc.
Two, some people are slash fans first, with favorite shows, and some people are Pros or B7 fans, with an extra interest in slash:
- Someone who is a Pros fan first, will tend to want 1st wave stories, with lots of work on the milieu, think of them as 'straight' as they are in the canon, etc.
- Someone who is a slashfan first, will tend to be okay with PWPs and other stories that let them get right down to it; i.e., later wave stories.
Three, and most important: there are threshold fandoms.
So, someone in their threshold fandom could be a first waver for that fandom, but a 2nd or 3rd waver in ther fandoms. That is, for their first fandom, they are: totally caught up in it; loyal to aired facts over fan canon; reads both gen and slash in that universe, considers g-rated stories to be slash even if they don't include sex, justifies sex interest through universe details... (I think those are the major 1st wave points. Wouldn't you know I can't find my copy of the essay right now), but in their next fandom, they would be more 2nd or even third waveish, that is: found the fandom through slash stories, not the show, fan canon is as important as show details, will read slash stories of shows they haven't even seen. Now these could be people that got involved in media fandom just in the last couple of years--way too late for the 1st wave, but they show all the signs in their 'first fandom,' especially if they found it pretty much by themselves (i.e., starting writing their first Sam/Al story before they'd ever heard of media fandom or slash.)
- In the US, very few fans found Professionals fandom on their own, since nearly the only way to watch Pros is be handed a tape by another fan. So, there are few people for whom Pros is their first fandom. On the other hand, lots of people started in fandom because they, all on their own, became a ST fan. Beauty and the Beast and QL are the other two big threshold fandoms of the last few years. Fans in threshold fandoms are more likely to be new; are more likely to have only new fan friends, and are more likely to be totally loyal to their current show.
Applicability For Today's Fans
When it was first released, the Wave Theory produced significant discussion from fans who felt it did - or did not - apply to them or their particular fandoms. However, as fandom has grown exponentially on the Internet the 'lines' between wavers have blurred as fans jump into and out of fandoms more quickly. The better example might be a model in which concurrent 'waves' spread out from multiple locations to be absorbed simultaneously (or not absorbed) depending on the content of the wave.
I prefer the "threshold fandom" theory described on the same page. But I enjoy it as a Theory of Fandoms rather than Theory of Slashers. I definitely feel like there is slash written now - not all slash and not necessarily good slash or bad slash - that engages more with slash tropes and ideals than it particularly does with the original text. (For example, I would say that Arthur/Eames, to me, draws on the movie characters to tell slash stories, it doesn't use slash to tell stories about the movie characters. There are some exceptions but yeah.) Whereas I do feel like 30-year-old fic has a particular flavour that is not much like modern slash and does sound a lot like the so-called first wave. It doesn't at all describe an individual fan's journey (and it's illuminating that it's written from a self-described earlier wave slasher) 
What If The Waves Were Places And Not Stages?From Old Fandom, New Fandom, and melodrama by elspethdixon, dated Dec 7, 2006:
From the grand Bitchy Old Fan wisdom generated by five years in fandom, with the aid of a friend who’s spent ten years in fandom, I present a slightly different categorising system.
First Wave Fandom: Fanfic what was in zines.
Second Wave Fandom: Fandom what was on mailing lists.
Third Wave Fandom: Fandom what is on livejournal.
These waves aren’t by any means mutually exclusive—zines and mailing lists are still around, as are many fans who got their starts in zines or listservs—but the way fanfic is shared and disseminated and the way people enter fandom has changed with each “new wave.”
Now, I’m about to go one about some fannish history that doesn’t always involve me, and start making sweeping generalisations, so be aware that I may be theorising with insufficient data.
Back in the days of zines, you had to be a seriously dedicated fan to even discover that fandom existed, let alone to write for it. And reader response was much slower and involved actual with-a-postage-stamp mail—and, due to the nature of zine publishing, long fics tended to be published in one piece, not serially. Zine fic, judging by the scads of old Starsky & Hutch and Real Ghostbusters fic I’ve read, leans heavily toward longer (30 pages or more) fics with either a classic slash first time plot, or an action gen plot (or an action plot and slash, something SH fic is particularly good at delivering). And hurt/comfort. I swear to God, every zine fic I’ve ever read that wasn’t a parody has involved some level of hurt/comfort. It’s glorious. And also All About The Character Squee, and the OTP, and the emotional gratification. People don’t go to the effort to produce and distribute a zine—much less shell out actual money for one—unless they’ve got a certain level of investment in the fandom, or in the kinks (like h/c) that first wave fandom catered to. And zine fandoms, as far as I can tell, seemed to have a very tightknit community, which only makes sense in a world where other fans need to know your physical mailing address.
From cschick's comment further down the thread: In between the zines and the mailing lists, there was usenet. From 1993-1996, I'd say, you'd have been almost guaranteed to find a newsgroup for your fandom--probably 2, one for discussion, one for fan fiction--but not necessarily a mailing list. Mailing lists were expensive propositions until 1996/1997--they required (a) owning a server and a pipe, (b) paying a ISP quite a bit of money for the list, or (c) being "in" at a university that had a server that provided free mailing lists. I remember some lists that existed pre-1997/1998 bouncing from server to server, searching for one that would continue to support them. Then came egroups.
Usenet had a different attitude than the mailing lists--they were more of a free-for-all, because you didn't have a list owner or server admin to answer to, especially in the alt.* hierarchy. Many people migrated to mailing lists when they became more widely available because they couldn't stand the attitude--the fact that there was no one in control--of usenet.
With the advent of mailing lists [ETA: And newsgroups like alt.startrek.creative and that X-files one], fandom became more accessible, as well as faster (you can get feedback overnight, none of this Letter of Comment stuff!), and posting WIPs as they were being written became feasible—and hey, presto, reader response can begin actually shaping the creative process as a fic is being written (which I understand is what made some of the Sentinel smarm so over-the-top). Fandom also got a heavy influx of younger fans, since it was now online for free and you no longer had to travel to conventions and fork over cash to get fic. I’ve been on several mailing lists, and there were always several on-going “serial” fics being posted, again with scads of melodrama and action, if not always as much h/c as I could wish (except in Magnificent Seven Fandom. Mag7 has loads of h/c, lots of action-y gen plots, and is the best fandom on earth). And since many lists are character or pairing specific, there was also a heavy emphasis on OTPs and OTCs. Mailing lists and groups, as far as I am aware, are also the place where headers/labels were popularised. Zines have always had a certain level of content description, but I don’t remember seeing the sort of formal Title: Pairing: Warning: etc. format headers in older zines. And on a mailing list, which generally has a moderator, like most lj comms, there’s usually a set of rules one must conform to, or risk getting banned.
With lj, what fandom seems to me to have gained is connectedness—via friendslists and metafandom and fandomwank and a host of other multifandom forums, it’s now possible to be exposed to other people’s fandoms without going out and looking for them. There’s always been ties between various fandoms (multifandom conventions like MediaWest, multifandom zines put out in smaller fandoms, people who wrote for several tv shows…) but now it takes a lot less effort to be a fannish butterfly. When you subscribe to a yahoogroup, you aren’t always aware of what’s going on on other yahoogroups. Also, with speedy audience-response now an ingrained part of fandom, and with less effort involved in the posting/finding/reading of fic, shorter fics and ficlets have taken off.
What it’s lost, in some circles, is the sense of community that belonging to a mailing list—much less attending cons and knowing one another in person—can foster. There are people in fandom now who truly don’t view themselves as members of a community, and when they interact with people who do think of themselves as part of a fannish whole, misunderstandings occur (witness the debates over the need for warnings and pairing labels, with one side saying “but it’s only courteous, Ray,” and the other side saying “Fuck courtesy, Fraser. Let’s just kick them in the head.”). Also, I think there are a lot more of what I would call casual fans now; people who enjoy and appreciate fandom as a whole, but who may not have quite the same level of dedication to a particular fandom/set of characters that is needed in order to, say, write Man From Uncle fic for forty years after the show ended. People who are here more for the intellectual stimulation than the emotional gratification and squee (not that first and second wave fandom didn’t have intellectual fans, and not that there aren’t deeply emotionally invested third wave fans).
Also, with the advent of lj, where communities are still moderated and, well, communal, but one’s own lj is one’s own personal forum, people don’t have to follow the same sort of rules that mailing lists required (not that they always follow/followed them on yahoogroups or other lists, which have their own share of flame wars and wank). Some people may feel more able to share their opinions and thinky-thoughts, some may forget that other people are capable of seeing their non-locked posts and say things they probably shouldn’t have. And meta has really taken off and become popular and prevalent. Which I enjoy, being an English major at heart despite the History/MLS master degree I’m chasing.I’m somewhere between a second and a third-wave fan, but as a hard-core h/c junkie (I had a h/c kink as far back as elementary school, when I was a wee little girl who hadn’t even begun to care about slash or het or sex of any sort), I really like the type of fic first wave fandom seems to have so frequently produced. I love 300k h/c epics. I like action-y gen plots. I adore happy endings and emotional catharsis and melodrama. God I love melodrama, whether it’s Captain Blood or Tosca or Flamingo’s “Total Eclipse of the Heart.” Depressing, “realist” stuff like Kat Alison’s “End of the Road,” or wacky, OOC SGA crackfic leaves me utterly cold, but characters dying in each other’s arms, Elizabeth kissing Jack and then chaining him to the Black Pearl’s mast, Doc Holliday and Wyatt Earp striding toward the OK Corral with their coats flapping in the wind, Enjolras and Grantaire dying on the barricades, Sawyer trekking through the jungle with a septic shoulder wound, Kate shouting that she loves Sawyer to save him from torture, Wesley and Buttercup’s final kiss, torture, fighting, fencing, escapes, True Love, miracles… that’s what my fannish squee is all about (actually, that’s what I not so secretly think all fiction should be about). I could care less about streamlined, post-modern, “underwritten” stuff, regardless of how clever it is. Unless it’s written by Neil Gaiman, I’m unlikely to read that sort of thing more than once. For me, fiction is all about emotional gratification, whether it’s fanfic or 19th century novels or movies or opera, and I have no shame about it. This, I understand, makes me something of an anomaly among newer, third wave fans. 
- The Wave Theory of Slash Fandom, original version from 1993, accessed November 16, 2008; reference link
- more on "wave theory" in slash fanfic post by princessofg, 14 July 2008. (Accessed 23 Jan 2012); reference link; reference link .
- "I think possibly my version could use more than four waves. Maybe I'll just call it a spectrum." in Pairings, wave theory, interpretive communities post by torch, 22 March 2007, accessed September 21, 2009; WebCite.
- executrix's Meta: Drowning in Waving, dated July 14, 2008; WebCite. Also unovis's post Explanation of waves theory in slash dated July 15, 2008;WebCite.
- More '...from the vaults' stuff post by sherrold, Oct. 22nd, 2008; reference link.
- anonymous comment at fail-fandomanon dated November 21, 2011; reference link.
- anonymous comment at fail-fandomanon dated November 21, 2011; reference link.
- from Old Fandom, New Fandom, and melodrama by elspethdixon, dated Dec 7, 2006, accessed June 5, 2013/reference link
- WebCite for flambeau's post.