The Princes Risborough card key for the identification of hardwoods has been in use for almost forty years. As first described by S. H. Clarke, it comprised sixty-four anatomical features and fourteen others, to cover geographical origin, colour, weight, etc.; but, with new knowledge and in the light of experience, it was modified and some additions made in 1961. No further change has been made since but it has been in daily use and some observations are given on the confidence with which some of the selected features can be used. In preparing the key for publication, it was sometimes necessary to make comparative studies of groups of timbers, often including others than those finally incorporated in the key, which was restricted, for publication purposes, to woods of commercial significance. In this work, not only the diagnostic but also the taxonomic value of some features was reviewed and comment is made on this. Examples are given of variability within units of classification; some causes for this are suggested, and examples cited where the evidence of wood anatomy is of special interest in respect of current classification.