In standard snooker, 15 red balls are placed on the table at the start of a frame.
The reds are arranged in a triangular pack at the end of the table opposite the baulk area. Players try to pot a red before playing for a colour, repeating that pattern while reds remain. Each potted red scores one point and is removed, whereas colours potted during the red phase are normally returned to their spots.
The 15-red format is the basis of the traditional game and helps explain why a full frame can contain many scoring visits. After the last red is potted, the colours must be cleared in ascending value, from yellow through black.
Some forms of snooker use different numbers of reds, including six-red snooker, but that is a separate variant. The standard professional game described in the main rules uses 15 reds, not the six-red format used in some shorter competitions.