probably the place to ask this...

[Dukeof Kircaldy]

Is there a difference between a tartan and a plaid pattern (as opposed to a plaid garment)?

[Navyguy]

All tartans ar plaids, however not all plaids are tartans.

[Dukeof Kircaldy]

All tartans ar plaids, however not all plaids are tartans.

yes, but what is the difference?

[Bob C]

All tartans ar plaids, however not all plaids are tartans.

There's a lot of really bad information in this thread. Some of it joking, some not.

A plaid is a garment, not a pattern.

[Dukeof Kircaldy]

Depends on which definition you choose to use:

Main Entry: plaid
Pronunciation: \ˈplad\
Function: noun
Etymology: Scottish Gaelic plaide
Date: 1512

1: a rectangular length of tartan worn over the left shoulder as part of the Scottish national costume
2 a: a twilled woolen fabric with a tartan pattern b: a fabric with a pattern of tartan or an imitation of tartan
3 a: tartan 1 b: a pattern of unevenly spaced repeated stripes crossing at right angles
— plaid adjective
— plaid·ed \ˈpla-dəd\ adjective

[SteveB]

A tartan is a plaid where weft and wane are the same. However a fly plaid is a piece of tartan fabric. U.S. fabric shops love to market "tartan plaid", which is acrylic or cotton that has the same repeats on the weft and the wane, but are way out of proportion to each other. Our version of the Gold Bros. tat shop.

[Ryan Ross]

I'm not sure, but I think it's only a tartan if the warp and weft are the same (or nearly so, in the case of individual batches of tartan which have been woven to include a decorative band near the selvage).

[The Wizard of BC]

A Tartan is defined as a plaid like pattern where the Warp or vertical yarns and the Weft or horizontal yarns are arranged in the same sequence.
The colors of yarns are the same and the number of threads per color are the same.

Where the two colors cross it will then form a third color.

It is also accepted but not necessarily true that Tartan fabric should be woven in a Twill. Or where each warp yarn is woven over two Weft yarns and then under one weft yarn. This is a fairly new thing as the original fabrics were plain woven and did not have the distinctly diagonal pattern that a Twill does.

By definition the Welsh Tartans, although beautiful, are not true Tartans.
Also by definition a check where the two colors do not blend and form a third color is not a Tartan.

[Dukeof Kircaldy]

Does registration also make it a tartan? Or is that not necessary?

[The Wizard of BC]

If apprehended wearing a non-registered plaid and calling it a Tartan you will be subject to THE KILT COPS coming to your home, arresting your sheep from which fabric was shorn. The weaver who made it will be subject to a firm talking to, and you, the offender, will be made to stand on a public street corner in trousers with a big sign saying "Tartan Heretic". Next to you will be a bushel basket of steaming Haggis and all who pass by will be free to stuff them into your mouth till you repent.
For a second offense the punishment is too sever for me to relate on a family friendly forum.