The shape differed slightly by the holder or purpose of use; there was a set of rules stating that the emperor's shaku usually had a square shape for top and bottom ends, the shaku used during a Shinto ritual had a round shape for top end and a square shape for bottom end, the shaku used by retainers usually had a round shape for top and bottom ends, and the shaku used on celebratory occasions had a square shape for top end and a round shape for bottom end.
なお、使用者や用途によって形が微妙に異なり、天皇は通常は上下とも方形、神事には上円下方を用い、臣下は通常は上下ともに円形、慶事には上方下円の笏を用いることになっていた。