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A badge has a number of fannish functions and definitions.
Some badges are con badges and are what a fan wears at a convention. It serves a number of purposes. First, it assures convention organizers that the fan is a paying guest. Second, it lets other fans know one's name. And third, either by color or by other symbols, it lets other con-goers know one's function (con committee, special guest, security, dealer...).
Some badges offer additional information about fans. At one Harry Potter con, the badge was used to identify whether or not the fan had read the latest book, to avoid spoilers. Another fan writes to the zine, Discovered in a Letterbox and suggests that fans design a slash badge they can wear to help identify one another at gatherings. A fan remembers wearing a badge as a sign of support in 1982: "As soon as we saw those 'Fans Against K/S' badges, we made some of our own, viz: 'Fans Upholding the Concept of K/S!' and wore them with pride." Some badges given both for practical reasons, but also as a thank you for a service provided.
Badges in the Forever Knight fandom were a feature of Forever Knight Factions: "Nunkies Scouts are members of the Nunkies Anonymous faction who earn themselves badges by completing virtual tasks. One officially becomes a Nunkies Scout once the first badge is earned." 
Some badges were worn to identify members of a certain club to others. One fan club president urges discretion in wearing the Star Trek Welcommittee badge: "If someone' is going to be on a panel that discusses any other club, convention, or so-called "rip-off artist", we ask that you do NOT wear your STW badge, nor do you make comments in the name of the Welcommitee. It has been brought to our attention that just such a thing happened at a convention in New York last year, and it left a bad image of STW in the minds of a number of fans." 
Some fan clubs issued badges to fans as a part of their membership. One example is for the X-Files fan club, Conduit: "In addition to the newsletter, subscribers receive a laminated card of your choice (a named I.D. card with either Mulder, Scully, or blank photo; FBI Visitors badge; or a Pentagon badge). Additional members at the same address cost an extra UK pound (includes another badge of your choice)." 
A "badge" in the UK is a Button in the US.
Some Comments on Pseuds
In the past, con badges were imprinted with a fan's first and last name, usually their legal one. Con badges now may use a fan's pseud instead of their legal name.
- "Rachel Sabotini and Debra T. and I were on a panel I'd proposed on "Privacy and Community: Netnames, Pseudonyms, and Face-to-Face Meetings." It was frightening to me to do that panel; I wanted to talk about and explore (among other things) some of my own prejudices, like the one against pseuds and screen names that sound stupid or juvenile to me, and it's scary to do that, especially in front of people I don't want to insult but who have names like "Attila the HunEE" on their badges. I was relieved when Lezlie made clear that I wasn't the only person who doesn't like anonymous conversations, who feels somehow less comfortable with blatant pseuds in social use than with real or real-ish names." 
- "Later during that afternoon I began to realize the disadvantage of doing most of your fan stuff through a pseudonym (and omitting to write that pseudonym on your con badge!). People I'd been in a room with, talked to and drooled over vids with for the last 24 hours suddenly began to come up and say things like "so *you* are Solo?" Uh, yeah. Sorry guys. Will try to do better next time." 
- "Friday at 4:30 was the S&H panel, attended by many veteran fans of the boys, including codename: Britt(a? nametag obscured. and what is the point of having a secret identity if you end up wearing it on a badge on your chest?)..." 
The Badge - Other Comments
- One fan, at NolaCon, complains about the size of the badge: "I hope that Noreason's name badges are larger so that I don't have to stick my nose in someone's breast to see who the hell they are!" 
- A fan complains about some con badges and lack of attention to detail: "One point I would like to mention (purely a personal observation) is that future committees take care when designing badges for attendees. Terok Nor gave no information whatsoever about the significance of colour coding and numbers etc.." 
- A fan remembers Lucasfilm's supposed crackdown on dealers selling unlicensed merchandise, usually pictures and the like: "Once incident I recall was at IguanaCon, the Phoenix World Con in 1978. The FBI showed up at the Dealers Room and the badge checker at the door said in a LOUD voice, 'I don't care if YOU ARE THE FBI, you can't come in here without a membership!' Tables were hastily covered, and what the Feds couldn't see, they couldn't do anything about." 
- "Fans are a minority group. We have our own lifestyle, one that outsiders, mundanes, can't really understand. We constantly referred to derisively, even feared by some religious groups. I've been called a devil worshipper, and once at a con, a mundane in the elevator with me saw my con badge and said, 'Oh, you're one of those weirdoes, aren't you?'" 
- "As this was my first Con, I was impressed with with the diversity of the group and their friendliness. The con badge was all the introduction needed!" 
Gallery of Badges
a 1979 badge for a dealer at ZebraCon
a badge from Anglicon 1988
from 1976, Mini Trek Con
badge for Scorpio, 1988
from OKon, 1977
program and badge for The British Star Trek Convention
a 1988 badge from DSV
a badge from Mos' Eastly Con in 1985
from 1984, Space Trek
Star Trek Prospers membership card. It welcomes the fan on board "as a member of the crew. Star Fleet Command hereby acknowledges your galactic flying privileges." Club membership was not limited to New York state. This club member was located on the West Coast.
S.T.A.R. club card from 1973
convention badge for Equicon 1973
convention badge for Equicon 1974
convention badge for Equicon 1975
- from The K/S Press #18, 1998
- from List of Forever Knight Factions
- from A Piece of the Action #50, 1977
- Fan Club info
-  a 1998 Escapade con report
- a Zedcon con report, 2000
- Notes from the underground: BASCon (2005)
- from Comlink in 1988
- from a con report for Terok Nor (convention)|Terok Nor
- from Comlink 56
- from Comlink 47
- from a fan's comment regarding Star Trekon in 1980