Septs
Septs are families or branches of clans using a different surname from that of the chief they follow. The idea of the sept has become contentious - because too many clans are claiming names as septs without showing any reason or proof of the link - but properly understood the concept is a valid one, and some relationships between a septname and the clan claiming it can be explained and proved (though many cannot!). There are various ways in which genuine septs come into being:
            1. By choice: When families or small clans placed themselves under the protection of a more powerful chief - often the head of a clan descended from the same parent kindred. Examples would be the MacKenzie sept of MacMurchie (i.e. MacMurchadh) or Murchison; and the MacLean sept of MacRaing or Rankin. Many families associated with Clan Chattan are said originally to have joined the Confederation by placing themselves under the protection of Mackintosh; but most of them are now considered clans in their own right rather than - as they used sometimes to be termed - as septs of Clan Chattan (e.g. MacGillivray, MacQueen etc).
            2. By conquest: When the land of a family or small clan is taken over by a more powerful chief to whom they then owe allegiance as tenants - a legal relationship subsequently reinforced by familial ties of marriage and political protection & favour. Examples include the MacLeod sept of MacRaild/MacHarold on Skye; the MacDonald sept of MacEanruig/MacHenry in Glencoe; the MacDougall sept of MacLulich/MacCulloch in Lorn.
            3. When a family choose to use as their surname a patronymic or a nickname remembering the particularly powerful individual member of the parent clan from whom they descend. Examples include the Gunn sept of MacRob or Robson named for Robert one of the sons of George Gunn "the Crowner" (i.e. coroner) of Caithness; the Campbell sept of MacConnochie (Gaelic original MacDhonnachaidh) named for Donnchadh/Duncan Campbell of Inverawe; the Cameron sept of Taylor named for Donald mac Ewen An Taillear Dubh ("The Black Tailor").  This sort of sept sometimes grew into a clan in it's own right; the most obvious example being the branch of Clan Donald descended from Alastair Mor, brother of Angus Mor mac Donald of Islay, who became the Clan MacAlister.
            4. When a branch of the clan use a different - often an Englished - version of the usual clan surname. Examples would include the Davidson sept of Mackay in Easter Ross (from MacDhai); the MacNab sept of Abbot (Mac-an-Aba means son of the abbot); the MacMillan sept of Bell (for explanation of which see Surnames).
            Many common names (trade-names and names derived from physical characteristics for example) are associated with more than one clan and area origins may determine which clan affiliation is appropriate for any particular family. Lengthy lists of septname affiliations - many of which should be taken with a large pinch of salt - can be found in:
Frank Adam, The Clans, Septs & Regiments of the Scottish Highlands (Edinburgh 1908, 1970, 1975)
George Way & Romilly Squire, Scottish Clan & Family Encyclopedia (Glasgow, 1994 & 1998).