Lenition in Irish

There is a common process of change in languages where the pronounciation of certain consonants is softened or relaxed, and gradually one consonant is replaced by another. Such a process happened a long time ago in the ancestor of English, giving rise to correspondences like these:
 

Latin	English
cordis	heart
pater	father
tres	three

The original c- sound of Indo-European was softened in Germanic to h-; the original p- sound was softened to f-; the original t- sound was softened to th-. Notice that the f in father is pronounced with the lips, like a p, except that the contact with the lip is softened so that air can get out. Similarly, the th in three is pronounced much like a t, except that contact with the tongue is softer, allowing air to get out. Linguists call this softening "lenition", from the Greek word lenis "soft".

Here's the same table with the Irish words included:
 

Latin	Irish	English
cordis	croidhe	heart
pater	athair	father
tres	trí	three

In Irish, consonants underwent a similar softening, but only between vowels, not at the beginnings of words. The t in the middle of the word athair has been softened to th, just as in English three (the p at the beginning has simply been lost altogether). In Old Irish, this th would have been pronounced much like it is in English today, but in Modern Irish it's been softened further to a simple h sound.

The English word right is connected to the Latin word recte in the same way: the c was lenited and slowly became ch like Scottish loch or German Bach (spelled gh, though). Afterwards, the lenited sound itself was lost in speech, and the vowel was lengthened in compensation. Something similar has happened in the Irish word croidhe. The d was lenited, and the dh would have been pronounced like the th- in English there. But then it gradually ceased to be pronounced, and the vowel was lengthened, so that in Modern Irish the word is pronounced (and spelled) croí.

In Irish lenition is called séimhiú, which (believe it or not) means "softening". In old grammar books, it used to be called "aspiration", but that was technically incorrect and is no longer used. Lenition is shown in Irish by combining the original letter with the letter h, much as in English th, ch, ph, or sh. In English, ph stands for an f-sound, not for a p-sound followed by a h-sound, and in the same way, letter combinations with h in Irish stand for separate sounds, as shown in the examples below. So now you know why written Irish has all those h's strewn across it.

Lenition is very important in Irish because it's part of grammar. In English, this change of lenition took place historically, and is now over and done with. The same thing happened in the middle of words in Irish. But at the beginnings of words, lenition has become grammaticalised. That is to say, whether lenition is there or not depends on how the word is being used. For instance, the word for "a cow" is bó, but "the cow" is an bhó, with bh representing a lenited b, which has a v-sound. In the following tables, lenition is illustrated with verbs. The first form is a command ("do it!"); the second, with lenition, is the past tense form ("he did it"). Lenition is the only difference between these forms (at least for regular verbs). Two examples are shown for each letter, broad and slender, since all consonants in Irish are either broad or slender, even lenited ones. The letters not listed are not subject to lenition.

1. For the sounds made with the lips (b, m, p), the contact with the lip is softened with lenition, allowing air to pass out. Thus p-sounds becomes f-sounds, and both b- and m-sounds become v-sounds:
 

	Slender				Broad
Letter	Unlenited		Lenited		Unlenited		Lenited
b	bí	be	bhí	was	buail	hit	bhuail	did hit
m	meas	think	mheas	thought	mol	praise	mhol	praised
p	pioc	pick	phioc	picked	pós	marry	phós	married

 

2. Even though f is made with the lips, it's already softened enough to let air pass out. Under lenition, it ceases to be pronounced at all. In the past tense of verbs, an old particle do reappears before vowels in contracted form d'. Since lenited f is unpronounced, the verb form begins with a vowel, and so d' reappears here, too:
 

Slender				Broad
Unlenited		Lenited		Unlenited		Lenited
féach	look	d'fhéach	looked	fan	wait	d'fhan	waited

 

3. For the velar sounds (c and g), the contact with the tongue is softened, allowing air to get out. Lenited c thus sounds like it does in Scottish loch or German ach when broad and ich when slender. Lenited g is similar, but voiced. Historically, lenited d would have sounded like the th- in English there, but over time it came to be pronounced just like lenited g:
 

	Slender				Broad
Letter	Unlenited		Lenited		Unlenited		Lenited
c	ceap	think	cheap	thought	cuir	put	chuir	did put
g	géill	give in	ghéill	gave in	glan	clean	ghlan	cleaned
d	díol	sell	dhíol	sold	dún	close	dhún	closed

 

4. Lenited s sounds like h. Historically, lenited t would have been pronounced as in English three, but over time it changed, and now it, too, is pronounced h:
 

	Slender				Broad
Letter	Unlenited		Lenited		Unlenited		Lenited
s	séan	deny	shéan	denied	suigh	sit down	shuigh	sat down
t	teith	flee	theith	fled	tóg	take	thóg	took