Rehearsal
An open rehearsal is a rehearsal that the public can come and watch. In opera houses the dress rehearsal is often an open rehearsal. The entry may be free or the audience may have to pay, but not as much as for a ticket for a performance. The audience will have to understand that the performance may be stopped if something goes wrong and needs to be rehearsed. Singers will often save their voices by not singing as loudly as they will in the performance (this is called "marking").