Graiméar na Gaedhilge/Part I Chapter IV
Attenuation and Broadening.
30. Attenuation is the process of making a broad consonant slender. This is usually done by placing an i immediately before the broad consonant, or an e after it. Thus if we want to make the r of mór (big), slender, we place an i before the r; thus móir. If we wish to make the f of fad (the termination of the 1st person singular future) slender, we write fead &c.
31. Broadening is the process of making a slender consonant broad. This is often done by placing a u immediately before the slender consonant, or an a after it; thus the verbal noun of derived verbs ending in igh is formed by adding adh: before adding the adh the gh must be made broad; this is done by inserting a u; mínigh, explain; míniughadh, explanation. If we want to make the f of fidh (the termination of 3rd singular future) broad, we must write faidh. Buailfidh sé, he will strike; meallfaidh sé, he will deceive.
Whenever a slender consonant is preceded by an i which forms part of a diphthong or a triphthong, the consonant is usually made broad by dropping the i. Thus to broaden the l in buail, or the n in goin, we drop the i and the we get bual and gon. The verbal nouns of buail and goin are bualadh and gonadh.