The relation between the design and the behaviour of a clasp can be expressed mathematically in terms of two very simple equations. One of these, the S equation, indicates the strength of the clasp, while the other, the Q equation, measures its performance. Two of the factors in the Q equation, Qc and Qm, are contributions to the performance from the design and the material respectively. They are therefore figures of merit or ` quality factors ' by means of which the adequacy of the design and the material can be judged. Choice of the correct undercut and clasp thickness can be made quickly and easily with the Q and S design charts and two examples are given in the text by way of illustration. The likelihood of fatigue failure is discussed in the light of this analysis and, while it cannot be ruled out with the information at present available, the chief cause of failure would appear to be faulty design.