A classification of amino acid type is described which is based on a synthesis of physico-chemical and mutation data. This is organised in the form of a Venn diagram from which sub-sets are derived that include groups of amino acids likely to be conserved for similar structural reasons. These sets are used to describe conservation in aligned sequences by allocating to each position the smallest set that contains all the residue types brought together by alignment. This minimal set assignment provides a simple way of reducing the information contained in a sequence alignment to a form which can be analysed by computer yet remains readable.