The Buchanan sett seems to be a more complicated version of what is commonly called Ancient MacMillan. Now there may be any number of reasons for this ranging from pure coincidence to the addition of the white stripe (and the darker over cheque) being indicative of Buchanan being, or becoming, a sept of Clan MacMillan in the very late 17th or early 18th century.

In 1667 the Dunmore branch of the Macmillans (the chiefly line) first became related to the Buchanan's of Leanny through marriage. In 1682 the last chief of the Buchanan's died, and a few years later Duncan Macmillan of Dunmore married Katherine Buchanan of Leanny (his cousin and the representative of the senior branch of the Buchanans) thus uniting the two houses. In 1742 another Duncan Macmillan of Dunmore registered arms with the Lord Lyon, which, in heraldic terms, may be described as a composed coat coat of arms.

Without going into great detail, when two armigerous families are united in matrimony the children of that marriage quarter the arms of both the father and the mother, with the arms of the father being placed in the first and fourth quarters, and the arms of the mother's father being placed in the second and third quarters. However, as sometimes happens an entirely new coat is devised based on elements found in both coats of arms. This is especially true when the family of the mother becomes extinct in the male line.

The heraldry works like this:

Duncan (1) marries Katherine Buchanan of Leanny and uses the MacMillan arms.

Alexander, their son, quarters Buchanan of Leanny with MacMillan.

Duncan (2, son of Alexander) sues out arms in 1742 asserting his status as the inheritor of clan Buchanan in right of his grandmother, Katherine Buchanan of Leanny, as there are no remaining members of the chiefly line of Buchanan.

In composing this "new" coat of arms for Duncan Macmillan of Dunmore the Lord Lyon changes the colour of the field from white to yellow (the colour of the Buchanan arms) and alters the position of the lion from passant to rampant (again, as depicted on the Buchanan arms). The three blue stars of MacMillan are retained in chief, and this new coat of arms is confirmed by the Lord Lyon. In so doing the Lyon clearly shows that the chiefly line of the Buchanans is now merged with that of the MacMillans and that Macmillan of Dunmore is chief of the Buchanans in the same way he is also chief of the Baxters, Blues, Bells, and Browns, and all other septs of the clan.

While it is probably impossible to say with any degree of certainty whether or not the Buchanan tartan is based upon that of the MacMillans, and if it is when it was first adopted, the genealogical and heraldic facts stated above would tend to support the theory that Buchanan, having quite possibly become a sept of MacMillan sometime between 1682 and 1742 at some point adopted and modified the Ancient Macmillan tartan as their own.

I am sure someone out there will choose to disagree with this thesis.