Graiméar na Gaedhilge/Part I Chapter V
Caol le caol agus leaṫan le leaṫan;
Slender with slender and broad with broad.
32. When a single consonant, or two consonants which easily blend together, come between two vowels, both the vowels must be slender or both must be broad.
This is a general rule of Irish phonetics. It has already been stated that a consonant is broad when beside a broad vowel, and slender when beside a slender vowel; and also that the sounds of the consonants vary according as they are broad or slender: hence if we try to pronounce a word like fearín, the r, being beside the slender vowel i, should get its slender sound; but being also beside the broad vowel a, the r should be broad. But a consonant cannot be slender and broad at the same time; hence, such spelling as fearín, málín, and éanín, does not represent the correct sounds of the words, and, therefore, the device adopted in writing Irish is to have both the vowels slender or both broad; e.g., firín, máilín, éinín.
This law of phonetics is not a mere spelling rule. If it were, such spelling as fearaoin, málaoin, éanaoin, would be correct. But no such spelling is used, because it does not represent the sounds of the words. The ear and not the eye must be the guide in the observance of the rule "caol le caol ⁊ leaṫan le leaṫan."
Two consonants may come together, one naturally broad and the other naturally slender. When this happens, Irish speakers, as a general rule, give the consonants their natural sounds, i.e., they keep the broad consonant broad, and the slender one slender. For instance, the ṁ of coṁ is naturally broad, and the l of líon is naturally slender. In the word coṁlíon (fulfil), the first syllable is always pronounced broad, although the word is usually written coiṁlíon. This is an instance of the abuse of the rule caol le caol. There are many words in which a single consonant may have a slender vowel at one side, and a broad vowel at the other; e.g., aréir (last night), aníos (up), ariaṁ (ever), arís (again), etc.
Although the rule caol le caol had been much abused in modern spelling, in deference to modern usage we have retained the ordinary spelling of the words.