Chalk in the shape of a flat square (from 5 by 5 cm; thickness approx. 0.8 cm) block to properly mark fits. It comes in different colors (white, yellow, red and blue), does not rub off on the hands and can be easily tapped.
Sometimes the dressmaker's chalk is put in a chalk holder, with sharp edge to sharpen the chalk (1). [MOT]
(1) See MIDGLEY & LAWTHER: 280.