Compare analyses on classifying irregular verbs (Strong verbs) into several groupings according to their sounds.
Greenbaum, Sidney, and G. Nelson, 2002. An Introduction to English Grammar, Longman, 2nd edition, pp. 92-94.
Leech, Geoffrey, and J. Svartvik, 2003. A Communicative Grammar of English, Routledge, 3rd edition, pp. 218-222
Quirk, Randolph , S. Greenbaum , G. Leech, and J. Svartvik, 1985. A Comprehensive Grammar of the English Language, Pearson Longman, 2nd edition, pp. 103-120