Seven Keys to Baldpate
Seven Keys to Baldpate is a mentalism routine developed by Theodore Annemann in 1931 which he named after the title of an Earl Derr Biggers' mystery novel.
The effect uses a lock and several keys, only one of which will open the lock. The keys are mixed and selected by spectators and the mentalist devines which person is holding the key that will open the lock.
It was originally created by Kolar.
There are many methods and routines that have been developed over the years, including marketed versions that include gimmicked and ungimmicked padlocks.