In snooker, the semicircular area behind the baulk line is called the D.
The D is the part of the baulk area from which the cue ball is played at the start of a frame and after many fouls. Its curved boundary, together with the baulk line, gives the area a shape resembling the capital letter D. The cue ball may be placed anywhere inside the D for the opening stroke.
The three baulk colours are positioned on the baulk line. Looking from the baulk end toward the reds, the yellow is on the player’s left, the green is on the player’s right and the brown is in the middle. These spots form an important reference arrangement for both the table and the rules.
The D is also central to “in-hand” situations. When the cue ball is in hand, a player can place it within the D before playing. The ball must then be struck legally, and a player cannot simply move it outside the D before the stroke.