The homo-organic rule or When not to lenite

Every Gaelic textbook will teach you about lenition and when to expect it. They teach that there's lenition after feminine nouns, after the definite article in certain cases, and so on. These rules are "relatively" straightforward. But as we all know, there are certain exceptions to the rules so you have sgian mhór but sgian-dubh, an fhàinne but an duilleag, Dùn Bhreatainn but Dùn Dèagh, MacDhòmhnaill but MacCaluim, and the song goes mo nigheann donn. After such exceptions, you'll often find a list showing you that an does not lenite feminine nouns beginning with d or l, and that's followed with a statement that it just IS that way: Dùn Dèagh and not *Dùn Dhèagh.

Fortunately, there's an easy rule that Linguists call the homo-organic rule which involves "sounds made with the same organ". You could call it the Sgian-dubh Rule to make it easier to remember because the phrase, Sgian-dubh, demonstrates the rule in action. But, before we can understand this rule, we need to look at our mouth again and where we make sounds.

Broadly speaking, in Gaelic there are three important areas in your mouth where you make consonant sounds: at your lips (labial sounds), at your teeth (dental sounds), and at your velum (the place at the back of your throat where you would make a [k] sound).

Group 1	(labials)	b, p, m, f
Group 2	(dentals)	d, n, t, l, s
Group 3	(velars)	c, g

Why do we have this rule? Because the rule in Old Gaelic was that whenever you had two sounds in the same group coming together, lenition was blocked, even if the grammatical rule stated "Lenite here, please!". So, let's look at some examples:

Dùn Bhreatainn	n is in Group 2, b in Group 1 ⇨ Lenition	Dùn Dèagh	both n and d are in Group 2 ⇨ No lenition
Camshronach	m is in Group 1, s in Group 2 ⇨ Lenition	Caimbeul	both m and b are in Group 1 ⇨ No lenition
MacDhòmhnaill	c is in Group 3, d in Group 2 ⇨ Lenition	MacGriogair, MacCaluim	both c and g are in Group 3 ⇨ No lenition
sgian mhór	n is in Group 2, m in Group 1 ⇨ Lenition	sgian-dubh	both n and d are in Group 2 ⇨ No lenition
air an fhearann	n is in Group 2, f in Group 1 ⇨ Lenition	air an duilleag	both n and d are in Group 2 ⇨ No lenition
An Fhraing	n is in Group 2, f in Group 1 ⇨ Lenition	air an deoch	both n and d are in Group 2 ⇨ No lenition

Of course, things are a bit more complicated. In modern Gaelic this rule has started to break down, bigtime, so it's not always applied. You'll find that this rule is most strictly adhered to with place names, surnames, and after the definite article. This rule is most intact with dental sounds (Group 2) yet infrequently applied with sounds from Group 1 and 3.

So, as a pointer to good "current" Gaelic, we suggest you adhere to these rules with surnames, place-names, the definite article an, the negation cha(n), and certain verbal forms like bhios, bhiodh and bu, but not elsewhere. Here are a few examples of what sounds like "good Gaelic" and what sounds like "odd Gaelic" to native speakers:

Definite article
in the nominative case	an deoch, an luchag, an drochaid, an tunnag ...
in the dative case	anns a' Ghleann Dubh, aig an duine ...
in the genitive case	an dorais, an taighe, an leanna ...
bu	bu dona, bu tioram, bu salach, bu nimheil ...
cha(n)
+ future	cha tòisich, cha sàbhail, cha nochd ...
+ personal pronoun	cha tu(sa), cha sinn(e), cha sibh(se)
bhios + thu	a bhios tu
bhiodh
+ pronoun	b(h)iodh tu, b(h)iodh sinn, b(h)iodh sibh
Surnames	MacCaluim, MacGriogair, MacCriomain, MacGilleEathain ...
Placenames	Dun Dèagh, Bad Darach ...
Frozen Forms*	sgian-dubh, nighean donn ...

* A frozen form is a phrase or word which preserves some feature which is no longer salient in the current form of the language. All languages have frozen forms. For example, English has frozen plurals for certain nouns such as mouse - mice, goose - geese. These are remnants of an old way of forming the plural which is no longer active in modern English. Compare that with German, where the "Umlaut" is still very alive: Haus - Häuser, Maus - Mäuse, Gans - Gänse. And Gaelic has frozen plurals forms such as mac - mic, balach - balaich, eun - eòin, and so on.

Now you might have noticed that all the words above that are blocking lenition end in dentals (chan, bhios) or sounds which used to end in a dental (bhiodh). But what about bu? Good question. Something VERY ancient is going on here. In Old Irish, this used to be bad and up to the Irish spelling reform, this was written as badh or budh, in Irish. In Gaelic, this hasn't been written that way for a long time and the spoken sound had disappeared even before it was eliminated in writing. Nevertheless, the effects of this ancient -d form is still present in modern Gaelic.

Even though sgian-dubh is acceptable, because it's a frozen form, other phrases like *ad donn rather than ad dhonn have come to sound odd to native speakers and should be avoided.

While it does not necessarily concern you as a learner, i's an interesting issue for language planners when it comes to fixing orthography or forming neologisms. Which of the following options should be considered for standard usage - Sgoil Ghàidhlig Glaschu or Sgoil Ghàidhlig Ghlaschu?

To a certain degree, this even applies to the lenited variants of these sounds, but only very infrequently, e.g. ath-thé vs. ath-té.

The reason for this rule is most likely laziness – if you have to make a d at your teeth and a dh at your velum, in quick succession, your tongue has to move around a lot – and your tongue is a lazy thing. Linguists debate this 'ease of articulation' concept - but it helps to make sense of this particular one.