In the world of home climbing walls, ledgers are 2 x 4s that are mounted horizontally to a wall by screwing them to the studs in the wall. These allow for more attachment point for the panels since the panels are screwed to the ledger and not into the studs which have a specific spacing. It also lets you use virtually any handhold with a bolt that is too long so that it pokes back behind the panel when completely screwed into the hold. If the bolt is too long and bumps into the wall directly behind the panel, you will end up with spinning holds because you can’t tighten them down. The ledger provides enough space for most bolts to be completely tightened without hitting the back wall.