The reamer is constructed from a square or triangular blank, machine twisted into a spiral but with fewer cutting flutes than a file. The reamer will only cut dentine when it is rotated in the canal; the mode of action described for its use is a quarter to a half turn to cut dentine, and withdrawal to remove the debris. The stiffness of an instrument increases with each larger size, so that larger reamers in curved canals will tend to cut a wider channel near the apical end of the root canal (apical zipping). Considerable damage may be caused to a root canal by the incorrect use of a reamer, and their routine use is no longer recommended.