The main component of a tartan pattern is the under check. This is
formed by the larger areas of colour that underlie the over check of
smaller lines and bands.
The under check is often an illusion of the eye because the over check breaks
it up and the eye then puts it back together.
If we ask when an over check is an under check, the answer ultimately
rests with how the eye sees the pattern, but most patterns have
a simple under check and seeing it can unlock the design.
The simplest under check is the two colour design,
usually of equal size where little or no over checking is used.
Examples of plain designs are the Shepherd Check (dark and light),
the Rob Roy (red and black) and the Moncreiff (red and green), shown
A third colour can be introduced, at the border of the two pivotal
under checks, to create a three colour pattern. The most widespread
use of this is the Black Watch family, of which there
are hundreds, in which blue and green has black placed between.
There is a tendency for this third colour to be thinner and form
a thick "over check" when left plain.
Identifying the under checks is a good way of simplifying
the pattern and locating the pivotal points in the design.