
The grain of a fabric is determined by the alignment of the yarns in a woven fabric. More precisely, the grain line is determined by how the warp yarns run.
Woven fabric are produced by weaving the so-called weft between the warp threads. The warp threads run along the length of a fabric roll, parallel to the selvedge, while the weft runs back and forth across its width.
Grain is indicated on a pattern by the grain line. When cutting out your pattern, it's important to align the grain line of the pattern to the grain of your fabric.
> For more warp and weft details: [Weaving on Wikipedia](http://en.wikipedia.org/wiki/Weaving)