In both editions of the author’s ‘Families of Flowering Plants’ was a key for identification of families of the world. This was constantly improved by the author and has now been edited separately as a companion to his new work ‘The Genera of Flowering Plants’. Essentially the structure of the key is not changed. It is made more handable by having instead of 15 groups in former editions, 32 groups in this work. The number of families, already risen to 411 in the 2nd edition of the 'Families', is still increased by some new ones in this key, and new families appear steadily in the 'Genera', so that we will soon end up with half a thousand families of which more than 60 % will be monotypic or almost so.