In English spelling, the three-letter rule,[n 1] or short-word rule, is the observation that one- and two-letter words tend to be function words such as I, at, he, if, of, or, etc. As a consequence of the rule, "content words" tend to have at least three letters. In particular, content words containing fewer than three phonemes may be augmented with letters which are phonetically redundant, such as ebb, add, egg, inn, bee, awe, buy, owe, etc. Vivian Cook says of the rule, "People who are told about it are often surprised that they were previously unaware of something so obvious."
Many content words would be homographs of common function words if not for the latter's "redundant" letters: e.g. be/bee, in/inn, I/eye, to/two. Otto Jespersen suggested the short spelling was a marker of reduced stress. Content words always have at least one stressed syllable, whereas function words are often completely unstressed; shorter spellings help to reflect this. (Interjections such as ah, eh, lo, yo are always stressed. Punctuation serves to isolate these elements.)
The short word rule dates from the Early Modern English period. In Old English, inflections increased the length of most content words in any case. Through to the seventeenth century, before English spelling was firmly settled, short forms for some content words did occur, such as eg (egg), ey (eye), lo (low), etc. Conversely, poets alternated between short and long forms for function words, depending on whether they occurred on or off the meter. Some commentators have ascribed such a convention to John Milton, although others suggest that it was unevenly implemented and clouded by intervention from the printer.
While many function words have more than two letters (and, she, were, therefore, etc.), the exceptions to the rule are rather two-letter content words. Only a few of these occur commonly in most texts: the words go (which also has a functional usage in the idiom going to do something), ox and, especially in American texts, ax.
English grammar is relatively flexible about converting words of one class to another, allowing verbal uses such as to up the ante or nominal uses such as the ins and outs. The verb forms be, am, is and do can be considered exceptions when used as lexical verbs, which are content words, though not when used as auxiliary verbs, which are function words.
Many recent loanwords retain spelling from the source language or are romanized according to non-English phonetic conventions. This has resulted in short words such as the notes of the solfège scale (do, re, mi, etc.;[n 2] from Latin via Italian) or the Greek alphabet (pi, nu, etc.) and miscellaneous others such as bo, qi, om, and ka. Carney calls such words "exceptions which prove the rule, clearly marked as exotic by the spelling".
- Stubbs, Michael W. (2011). "Spelling in society: Forms and variants, users and uses". In Tracy, Rosemarie (ed.). Who Climbs the Grammar-Tree: [leaves for David Reibel]. Linguistische Arbeiten. 281. Walter de Gruyter. p. 223. ISBN 978-3-11-163382-4.
- Cummings, D. W. (1988). "The Short Word Rule". American English Spelling: An Informal Description. JHU Press. pp. 87–89. ISBN 978-0-8018-3443-1.
- Carney, Edward (1994). "3.2.5 The 'short-word rule'". A Survey of English Spelling. Routledge. pp. 131–134. ISBN 0-415-09270-1.
- Jespersen 1961 §4.96
- Cook, Vivian J. (2014). The English Writing System. Routledge. p. 57. ISBN 978-1-4441-1901-5.
- Jespersen 1961 §3.134
- Jespersen 1961 §4.97
- Beeching, H. C., ed. (1900). "Preface". The Poetical Works of John Milton. Oxford: Clarendon Press – via Project Gutenberg.; Neumann, Joshua H. (1945). "Milton's Prose Vocabulary". PMLA. 60 (1): 105 fn.12. doi:10.2307/459124. ISSN 0030-8129. JSTOR 459124.
- Creaser, John (1983). "Editorial Problems in Milton". The Review of English Studies. 34 (135): 279–303. ISSN 0034-6551. JSTOR 517241.
- Shawcross, John T. (1963). "One Aspect of Milton's Spelling: Idle Final "E"". PMLA. 78 (5): 509. doi:10.2307/460727. ISSN 0030-8129. JSTOR 460727.
- Schönefeld, Doris (2005). "Zero-derivation–functional change–metonymy". In Bauer, Laurie; Hernández, Salvador Valera (eds.). Approaches to Conversion / Zero-Derivation. Waxmann Verlag. p. 132. ISBN 978-3-8309-6456-8. Retrieved 16 October 2020.
- Rainbow, Bernarr (4 October 2008). "Glover, Sarah Anna (1786–1867), music teacher". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/45795. (Subscription or UK public library membership required.)