When looking at groups of organisms, shared characteristics (homologues) provide the raw data from which hypotheses of common ancestry may be suggested. In order to explore the relationship between homologues and particular hypotheses of common ancestry, complex matrices are devised, where homologues are coded, allowing theories of homology to be developed and tested. Practically nothing has been written about this matrix-building process, which is fundamental to our understanding of diversity and evolutionary history. This book fills the gap by discussing the ways observations are coded and the consequences for resulting hypotheses using case studies and theoretical examples.