Modern English deadjectival nouns ending in -th (length, mirth, warmth etc.) are examples of a closed and unproductive word-formation set whose members have, from a diachronic perspective, frozen into lexicalisations. Sloth is no derivative of slow for present-day speech feeling. In warmth, -th is analysable, but the suffix cannot be added synchronically to an adjective to provide a noun (cf. Bauer 1983.48-9, with examples such as *psychedelicth, *surrealth).What follows is an attempt to explore some of the historical prerequisites of this lexicalisation process. Its roots must be sought in Old English where -th,‘a substantival suffix of doubtful currency’ in Modern English (Marchand 1969.349), still ranked, in its corresponding Old English forms, among ‘common affixes’ (Quirk and Wrenn 1957.114, 118). The lexicalisation process is likely to have been complex and, owing to the nature of linguistic evidence we have of (Late) Old and (Early) Middle English, difficult to reconstruct in its entirety.