Rehearsal marks label the sections of a song. In the Editor, tap or click the first measure of the section, then choose a letter.

 

iOS, Android: tap the rehearsal-letter button in the bottom row, then choose from the popup. Mac, Windows: click the rehearsal mark you want. The buttons toggle, so choose a highlighted letter again to remove it.

 

in (intro) and V (verse) mark a section that plays once at the very beginning. Use one of them only once, and follow it with an A so the player knows where later choruses begin. Many styles change feel between the A and B sections, and a drum fill usually lands at the transition.

 

For the full guide, see Edit a song on the iReal Pro website.