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, as 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 could call 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 (velar sounds, made at the place at the back of your throat where you make a <k> sound):

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

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, so lenition takes place
Dùn Dèagh – both n and d are in Group 2, so no lenition

Camshronach – m is in Group 1, s in Group 2, so lenition takes place
Caimbeul – both m and b are in Group 1, so no lenition

Mac Dhòmhnaill – c is in Group 3, d in Group 2, so lenition takes place
Mac Griogair, Mac Caluim – both c and g are in Group 3, so no lenition

sgian mhór – n is in Group 2, m in Group 1, so lenition takes place
sgian-dubh – both n and d are in Group 2, so no lenition

air an fhearann, An Fhraing – n is in Group 2, f in Group 1, so lenition takes place
air an duilleig, an deoch – both n and d are in Group 2, so no lenition

Of course, things are not quite that uncomplicated. In modern Gaelic, this rule has started to break down and is thus not always applied. The 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, it is suggested that 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.