Liquid consonants - shown in Gaelic as ll, nn, rr and m - in single syllable words or in syllables following a stressed vowel, as in 'calltainn' (hazel), have usually undergone a change from the original long consonant (shown on the maps by a colon). In Islay, Jura, Gigha and parts of west Argyll the original pronunciation has persisted, with 'chùm' extending even further into Argyll.

In most parts of the Gaelic speaking area, two main changes have happened. In some cases, such as in most of the northern area, including Speyside, the lengthening has been transferred from the final consonant to the vowel. In the case of 'chùm', this incudes Speyside. The second change, diphthongisation, is found in Lewis and on the west coast of Sutherland. The patterns for 'dall' and 'clann' are remarkably similar covering most of the Gaelic area apart from Islay, Jura, Kintyre and Arran.  Arran, in common with Speyside, adopted the pattern where the stress has been transferred from the consonant to the vowel. The Speyside version, however shows tendency towards diphthongisation, with pronunciation approximating to clo:in rather than cla:n.  In all three words in Perthshire there is a tendency for neither lengthening nor diphthongisation to have occurred. 

With 'chùm', five cases were seen where both the vowel and final consonant were lengthened.  These were given a category of their own (asterisk) and appear to be at the margins of the cu:m zone.  Only a very few similar uses were found for 'dall' and 'clann' - these have been omitted.

A note is to be found in the Appendix on classification of the lengthened syllable category.

Information on the this topic gained from Dr Seamus Grannd, Aberdeen University