My best guess:
Assuming most of the pieces are non-standard,
A “yard” is 36 x 60 = 2160 (inches?)
So, if my math is right:
1/4 = 540 1/2 = 1080 2/3 = 1440 3/4 = 1620
And your 17 x 42 piece = 714. 714/2160 = 0.33 = 1/3 of a yard
Wow, thanks.