Is buffalo plaid a tartan?

No. It is a two-color balanced check with no registered sett or affiliation, although its red and black palette descends from the MacGregor district checks. It is plaid in the American sense only.

What exactly is a sett?

The thread-by-thread recipe of a tartan: a sequence like B24 K4 G24 naming each color and its width in threads. The sequence usually reflects around pivot stripes and repeats identically in warp and weft, which is what makes a tartan reproducible.

Side by side

Tartan vs Plaid

In North America the words are used interchangeably, but they name different things. A tartan is a specific crisscross pattern defined by a thread count called a sett, historically tied to Scottish clans, regiments, and districts. Plaid is the umbrella word for any crossing-stripe pattern, and in Scotland a plaid is not a pattern at all but a garment, the long cloth worn over the shoulder.

Black Watch

No. 016

2/2 twill · first documented 1725

Buffalo Check

No. 002

plain weave · first documented c. 1850

The differences

Aspect	Black Watch	Buffalo Check
Definition	A registered or documented sett: an exact sequence of colored thread counts, identical in warp and weft.	Any pattern of crossing bands and stripes, tartan included.
Identity	Carries affiliation: clan, family, regiment, district, or institution.	Carries none; buffalo check belongs to nobody.
The word	From French tiretaine via Scots, attached to the patterned wool itself.	From Gaelic plaide, a blanket; the garment came first, the American pattern sense later.
Rule of thumb	Every tartan is a plaid.	Not every plaid is a tartan.

Which to choose

Use plaid freely for the pattern family and reserve tartan for setts with a name and a paper trail. If you can ask whose pattern it is and get an answer, it is a tartan.

Common questions

Is buffalo plaid a tartan?: No. It is a two-color balanced check with no registered sett or affiliation, although its red and black palette descends from the MacGregor district checks. It is plaid in the American sense only.
What exactly is a sett?: The thread-by-thread recipe of a tartan: a sequence like B24 K4 G24 naming each color and its width in threads. The sequence usually reflects around pivot stripes and repeats identically in warp and weft, which is what makes a tartan reproducible.

Full entry: Black Watch Full entry: Buffalo Check

Sources & References

1.Black Watch, Wikipedia
2.The Scottish Register of Tartans, National Records of Scotland
3.Check (pattern), Wikipedia
4.Mackinaw cloth, Wikipedia