Q. Does CMOS have a rule for using one el or two in verbs ending in “ing”? For example, “traveling” or “travelling”? “exceling” or “excelling”?
A. If you’re an American traveler who’s traveling in the United States, use one l; if you’re a British traveller travelling in the UK, use two. But if you plan on excelling in either region, you’ll want to use two l’s for that word.
That’s because of two competing conventions: (1) When -ing or -ed (or -er) is added to a word of two or more syllables that ends in a consonant preceded by a single vowel (like travel and excel), the consonant is usually doubled if the stress falls on the final syllable. Accordingly, you’d write traveling and traveled (because the stress in travel is on the first syllable) but excelling and excelled (the stress in excel is on the second). But (2) travelling and travelled (and traveller) are exceptions to the rule in British but not American usage.
We don’t know the reason for that exception, but we can offer as evidence this footnote from page 16 of the eighteenth edition of Horace Hart’s Rules for Compositors and Readers at the University Press, Oxford (published in 1904, the first year Hart’s guide was offered to the public):
We must, however, still except the words ending in -el, as levelled, -er, -ing; travelled, -er, -ing; and also worshipped, -er, -ing.—J. A. H. M.
That footnoted exception applies to the rule about not doubling consonants when the last syllable isn’t stressed. “J. A. H. M.” is James Murray, the main editor of the original Oxford English Dictionary, who is also credited on the title page of that edition (and others) of Hart’s Rules as one of two people responsible for revising its “English spellings.”
The three exceptions noted in the footnote above are still followed in British English. In American English, by contrast, level and (as we have seen) travel do not double the l, but worship usually doubles the p (as in British English).
CMOS used to include a list of preferred spellings—as on pages 37 and 38 of the first edition (published 1906), which showed traveler with one l and (in a decision backed by logic that nonetheless didn’t stick) worshiper with one p. CMOS now defers to Merriam-Webster for such decisions.