From the MIT hacking community. A sign-in is a small design or phrase used to indicate that a hacker has visited a location. Sign-ins are distinct from graffiti; where graffiti tends to be large, attention-getting, and frequently colorful, a good sign-in is only as large as it needs to be to get the point across. A hacker will sign in because he or she is proud of finding somplace. You wouldn't sign in under the carpet in Lobby 7; no one should be proud of finding one of the major landmarks of the MIT campus. A sign-in should not be located someplace where a passerby could easily wander in and find it. The inside of an air shaft, a tomb, or a hard-to-get-at ceiling in a machine room, however, are all good targets for sign-ins. Each hacker has his or her own sign-in, although there have been instances of groups all using the same design. The best-known sign-ins are probably No Toad Sexing, Bavicci Lives, and Sophocles.