Primarily the warp, ‘the grain’ of weave that runs the length of the piece. All patterns should be cut running with the grain–the arrow printed on a paper pattern should line up along the vertical grain. A textile’s strength, fall, drape and pattern have been carefully worked out by the textile designer to best advantage, so that the fabric works and hangs best when it’s used with the grain straight – i.e. vertical. Where the fabric needs to be cut off grain, on the cross or the bias, the pattern will indicate.
Occasionally the grain that the weaver wants you to use as your vertical, will be the weft, the horizontal yarn from selvedge to selvedge; in these cases some sort of note will be given with the sample, and with the fabric.
For quick reference, fabric cut on the cross, or bias, 45% off grain, takes advantage of the maximum flexibility for manipulating shape and form; fabric cut with the horizontal as the vertical railroaded) might be weaker or with a side nap, but you might save fabric and/or seam joins. Although the manufacturer will always offer advice, the decision is yours.