In many cases, the crosswise threads of a fabric have more stretch/give than the lengthwise threads of a fabric.
To check this out:
Take several pieces of different fabrics -
Measure out a ten-inch span (no need to cut it) - now pull it as much as you can - see how much it stretched - measure it to verify the number.
Now measure ten inches the other direction of the same fabric - stretch it and measure it.
Was there a difference?
(For good measure - you could also check how much stretch there is on the true bias of the piece.)
This is usually very obvious with one-way stretch knits.