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 ... 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 telling you that anan 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 need this? Because the rule in (old) Gaelic was that whenever you have two sounds which are in the same group coming together, lenition was blocked, even if the grammatical rule is saying "lenite here please!". 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 not quite that uncomplicated. In modern Gaelic this rule has started to break down bigtime and is thus not always applied. You will find that this rule is most strictly adhered to with place names and surnames and after the definite article. This rule is most intact with dental sounds (Group 2) and only 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 otherwise. Here are a few examples of what sounds like "good Gaelic" to native speakers and what sounds like "odd Gaelic":

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 not salient in the current form of the language any more. All languages have frozen forms - English e.g. 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 German, where this "Umlaut" is still very alive: Haus - Häuser, Maus - Mäuse, Gans - Gänse. Or Gaelic as a matter of fact - mac - mic, balach - balaich, eun - eòin etc.

Now you might have noticed that all those words above 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 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 for a long time and the sound had disappeared even before then but the effects of this ancient -d are still there.

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

While it does not necessarily concern you as a learner, it is an interesting issue for language planner when it comes to fixing orthography or forming neologisms - should it be 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.