You've read that the vast majority of tartans are symmetrical
but now we must deal with those that aren't. Basically the
difference is that instead of the sett pivoting and repeating
itself backwards as in the design on the left, the sett repeats
itself by starting at the beginning again as can be seen
on the right.
The thread count for the symmetrical tartan on the left is R/20
W20 B20 K/120 whereas the count for the asymmetric tartan on the
right is R20 W20 B20 K120 . . .
Because there are no pivots in an asymmetric tartan, none are
marked by the forward slash (/) and the fact that is asymmetric is
shown by the . . . which indicates that you just start again with
Different warp and weft.
Whilst we're dealing with exceptions to the conventional tartan,
we must also mention the situation where you can have a different
warp and weft. This is not recommended unless there's a pressing
reason for it such as the designer needing to incorporate more than
the conventional maximum of six colours. Once again, modern looms
distort tradition by being able to accommodate more than six but
good taste normally prevents that since most examples end up being
aesthetically challenged (a euphemism for 'ugly').
But there are occasions when a design brief can insist on more
than six and the best way of tackling that may well be by swapping
the extra colour(s) with some of the basic six and incorporating
them into the weft or warp alone. However, the width of the newly
introduced colours needs to be carefully judged and if the change
applies to a band rather than a line, then the effect can be
disconcerting and may well be unacceptable.
Another - and rather extreme - example of different warp and
weft can occur where the designer uses the same colours in both but
alters the thread count so drastically that the finished design is
hardly recognisable as a tartan. This is to be avoided at all