One origin of the spoke card was laminated cards inserted in spokes with numbers used to identify competitors in competitive races held by bicycle messengers, first in official competitions then in unofficial alleycat races. Another influence was the practice of placing playing cards found on the street in bicycle spokes in a game known in 1990s San Francisco as Spokepoker. Tarot cards were also used early on, with the race number written on them, but nowadays cards are often custom printed.
The spoke card is wedged between the spokes of the rear wheel at the point where they cross each other. Although the spoke card is hard to read while the bicycle is in motion, it provides a cheap way to label them in the absence of a proper race number plaque under the top tube, and is less likely to attract the attention of the authorities than a number on the rider's back. A spoke card can also be placed in such a way as to make a noise as the card flaps against the spokes.
Many messengers retain spoke cards after the event, accumulating several on the wheel. Other city riders sometimes fix spoke cards to their wheels as an affectation of messenger culture. Many times participants of large group rides, such as those organized by groups like the Midnight Ridazz or Critical Mass receive spoke cards and affix them to their spokes.
Spoke cards have evolved to serve such diverse functions as memorials for fallen messengers, as artworks, and as political placards in bike messenger association elections, and even in the 2008 United States presidential election.