On the July tartan of the month thread Peter MacDonald suggested I start a thread on this topic. Here is what I wrote on that thread:

I've read about a hypothesis that all or most of the tartans predating the Proscription can be divided (roughly by geography) into three or four types. Jamie Scarlett recognized three types: the Ross type worn north of the Great Glen, the Mackintosh type worn in the Grampians, and the Macdonald type worn in the west. I have a book that gives four types: Huntly, Glenorchy, Lochaber, and Lennox.
Peter wrote:

I'm broadly in agreement with the concept of regional groupings but not necessarily with Jamie Scarlett's classification. Historically I certainly don't recognise the four types you quote. The trouble is that previous writers have based their ideas on groups that include post c1780 setts in which I include: Huntly, Lochaber and Lennox.

I'm working on a paper that I hope will prove the existance of a generic Applin/Lorn/Lochaber setting based on several pre-'45 specimens.
The direct historical evidence for district tartans comes primarily from Martin Martin, who in 1703 wrote:

Every isle differs from each other in their fancy of making Plaids, as to the stripes in breadth and colours. This humour is as different through the mainland of the Highlands in so far that they who have seen those places is able, at the first view of a man's Plaid, to guess the place of his residence.
This has often been interpreted to imply a uniformity in the tartans worn in a particular region, but I think it more likely that it implied nothing more than a sort of "family resemblance" among the tartans woven in a particular area. There seems to be a rough consensus among tartan scholars that this family resemblance of regional tartans was later transferred to clan tartans, which might explain, for example, the similarities between the MacDonell of Keppoch and Mackintosh tartans.

I am in sympathy with the idea, but still a bit skeptical as to how much of the evidence we have about these groupings comes from before the Proscription era. An example of a case where the similarity between two tartans appears to date from after the end of Proscription is that of the Grant and Drummond tartans. Then there are the plaids supposedly given to Bonnie Prince Charlie by the chief of Clan X which were subsequently left behind in the castle of the chief of Clan Y....