When not to Lenite
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 <k> sound)
Group 1 (labials) b, p, m, f Group 2 (dentals) d, n, t, l, s Group 3 (velars) c, g
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!" 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.
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 sà bhail, 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 ... 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. 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. 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 These are remnants of an old way of forming the plural which is no longer active in modern English - compare German, where this "Umlaut" is still very alive. Or Gaelic as a matter of fact - mac - mic, balach - balaich, eun - etc.
What's a good name for a medieval Scottish person?