I'm a band teacher/musician and my primary instrument is percussion/drums. A LONG time ago, when AOL ruled the world, a colleague of mine, also a percussionist, used the AOL username of vibe 2100, so when I started a hotmail mail account I used perc as an abbreviation of 'percussion,' and stole my friends psuedo-arbitrarily chosen number of "2100." Since so many forums utilize email accounts, I usually just use "perc2100" as a username as an easy way to a) remember the username and b) remember which email account is associated with the forum. In more professional-setting forums and organizations I use my more formal emails, which typically revolve around my name, and I use the 'hotmail' account & username for the more fun forums.