THAT's what I was suspecting!
OK, so the back of a shirt is often cut in 2 panels, sometimes with a yoke across the shoulders. If that is the case, the two panels may be cut 'on the bias', so that any kind of squared pattern ('plaid'?) would form an interesting 'chevron' pattern at the central seam where they join.
Note that it does not say 'bias-
binding' but it
does say '
a bias', whereas 'bias-binding' is more usually uncountable in EN.
Here's the kind of thing I was thinking of, although in this case, it is the yoke that is cut on the bias — probably more common, when you think about it:
http://truebias.dreamhosters.com/wp-content/uploads/2013/08/...