The main component of a tartan sett is called the undercheck. These are the big background stripes. Most tartans have a simple undercheck of two or three main colors. The overchecks are thinner stripes that break up and modify the wide areas of the undercheck. Many tartans are identical except for the overcheck.

The Rob Roy (think lumberjack red and black check) is about the simplest tartan going - alternating red and black stripes of equal width. No overcheck at all. Add a narrow black overcheck to the red and a narrow yellow overcheck to the black, and you have the Wallace. Cunningham is also the Rob Roy undercheck with a more complicated overcheck.

The Black Watch and all its cousins have a three-color undercheck (wide blue and green separated by a narrower black). The reason that so many tartans look a lot like the Black Watch is that they have the Black Watch undercheck and some, but not all, of the Black Watch overcheck. For example, if you keep all of the BW overcheck but replace the narrow black overcheck in the green undercheck with a yellow overcheck, you have the Gordon instead of the Black Watch. If it's a narrow white instead of yellow, it's the Lamont. If it's a narrow white bordered by black (called a "guarded white), it's the Forbes. If it's a narrow azure, it's the Cheape. Other tartans added colored overcheck to the blue undercheck as well (Farquharson, Hunting Robertson, Murray of Atholl, etc., etc.). You could go on forever with Black Watch Family tartans.