Weft

The weft is the crosswise threads that run perpendicular to the lengthwise grain. These threads run at 90° angles to the length of woven fabrics (or the width). Also known as the cross grain it has little to no stretch and is usually 45° from the bias. (see also, warp and grain)
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Some fabrics have different fibers for warp and weft making an interesting play of texture in the fabric.

Silk dupioni often has different colors in the warp and weft. This makes for an iridescent quality in this luxurious fabric. Changes in direction and light can appear to change the actual color of the fabric.  This is a favorite fabric for vestments.   The colorplay, especially in low light, can be very effective.   It adds a bit of sparkle without appearing gaudy.