Typology crucially involves the investigation of data from a wide range of languages, and has become widely accepted as a way of understanding cross-linguistic diversity. However, an understanding of cross-linguistic diversity is in turn essential to a full understanding of the linguistic structure of a single language or of the structural differences between a pair of languages being studied contrastively. A number of illustrative examples are discussed, involving primarily data from English and Japanese against the background of the typological project The World Atlas of Language Structures.