Q: How do you tell the difference between tartan and plaid? —Ian H.
A: Ian, what a perfect question — and since we just celebrated "National Tartan Day" yesterday, your query couldn't be more timely. The date, April 6th, was established by Congress in 1997 as a day to recognize American citizens of Scottish descent. Being Scottish myself, it's always been a day to celebrate.
Okay, to answer your question: All tartans are plaid, but, not all plaids are tartan.
To keep things simple, I'm just going to stick to the basics and try not to overwhelm you with too much technical detail. All plaids and tartans are comprised of stripes (in varying sizes and colors) that meet at a 90-degree angle. We start heading into 'tartan' territory by looking at the geometry on the pattern. With most every tartan, the pattern on the stripes running vertically is exactly duplicated on the horizontal axis too. Basically, this matching pattern in both directions will create a grid. The warp and weft threads are then woven in a two-over-two "twill" pattern. When looking at a simple plaid, you'll notice that the stripes — either in color, size, or pattern — are not the same in both directions.
Take a look at the tartan details from my own home in San Francisco. Those are all classic tartans. You'll notice no matter the gauge of the pattern, the stripes meet to form a perfect grid in each example.
You can do more research on tartans at The Scottish Tartan Authority.