This is an issue I didn’t quite understand early on in my sewing. I thought that once the fabric is neatly pressed and flat, you are good to go.
What does it mean for Fabric to be on grain?
This issue usually affects woven fabrics and not knitted fabric. As I mentioned earlier, your fabric has a length-wise grain and a cross-wise grain.
The fabric is on grain when the Lengthwise grain is at right angles to the crosswise grain, or when the Warp threads are perpendicular to the Weft threads. In short when the threads of the fabric are straight. This is also referred to as Blocking your fabric.
If you fold the fabric selvedge to selvedge, the edges should be straight and at right angles to each other. Fabric will lie where it naturally will, so if you try to force it straight, you will have wrinkles.
The importance of being on grain is that it affects the drape or the fall of the finished garment, that is why your shirt is smooth and straight and your trouser legs are not twisted.
It also affects the alignment of stripes or patterns on the fabric that are made by weaving different coloured threads like the Flannel fabric above.
Please note that the pattern on Ankara and some other printed fabrics have nothing to do with grain lines. They are simply printed images on the fabric. As opposed to a fabric like Flannel or Check where the pattern is created by weaving different coloured threads together and therefore the straightness of the lines is directly related to the grain lines of the fabric