Once upon a time, going to a concert was 1) moderately affordable* and 2) always marked by one wearing an official t-shirt to school the next day. If it weren't clear who was going to a show before, everyone knew then by who was wearing the CSN&Y/Fleetwood Mac/Queen/Aerosmith/ELP etc. shirts. Or, in my case, Bee Gees.
Concert t-shirts were a signal of Cool.
In that spirit:
*My daughter just bought a concert ticket that was in the three-digits. Seriously.