Every Gaelic textbook will teach you about lenition and when to expect it; there is lenition after feminine nouns, after the definite article in certain cases and so on which are "relatively" straightforward. But as we all know there are certain exception to this rule ... 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" ... at which point you will often find a list telling you that an does not lenite feminine nouns beginning with d or l and/or that it just IS Dùn Dèagh and not *Dùn Dhèagh.
Fortunately, there is an easier rule. Linguists call it the homo-organic rule, the rule of "sounds made with the same organ". You can could it the Sgian Dubh Rule to make it easier to remember since it 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 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 Gaelic is that whenever you have two sounds which are in the same group coming together, lenition is 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
Dùn Dèagh both n and d are in Group 2 > Lenition
> no Lenition Camshronach m is in Group 1, s in Group 2
Caimbeul both m and b are in Group 1 > Lenition
> no Lenition MacDhòmhnaill c is in Group 3, d in Group 2
MacGriogair, MacCaluim both c and g are in Group 3 > Lenition
> no Lenition sgian mhór n is in Group 2, m in Group 1
sgian dubh both n and d are in Group 2 > Lenition
> no Lenition air an fhearann, An Fhraing n is in Group 2, f in Group 1
air an duilleig, an deoch both n and d are in Group 2 > Lenition
> 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, placenames, the definite article an, the negation cha(n) and certain verbal forms like bhios, bhiodh and bu but not otherwise. Here's 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 toisich, 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 ... Placenames Dun Déagh, Bad Darach ... Frozen Forms* sgian dubh, nighean donn ...
* A Frozen Forms is a phrase or word which preserves some feature which is not salient in the current form of the language anymore. All languages have frozen forms - English e.g. has frozen plurals for certain nouns such as . 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.
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.
