The angle between a defined plane or line through one atom (or group) and that through another connected atom (or group) depends on the rotation of one unit about the connecting arm of another unit. Since all valencies, angles and interatomic distances are predetermined in the manufacture of the units, it is only necessary to adjust the degree of rotation of one unit relative to the next to specify completely the position in space of each atom. Thus a polypeptide alpha helix is specified by two dihedral angles made by the planar amides on either side of an alpha carbon. A keyed protractor is supplied with the larger kits which fits over the square section of the arms, and enables any specified angle to be 'dialled'. The entire polypeptide backbone of a protein may therefore be dialled from a given list of dihedral angles. The backbone must be dialled in short lengths to maintain accuracy -these can then be checked with the coordinates -the ultimate criterion.