Grain lines are a generally unnoticed aspect of the garment, that is until they are either used in the wrong way and cause fit problem, or used in interesting ways to mould the fabric in different ways to the body. Sometimes they can also be used to turn a print onto a different angle for interesting visual effect.
When fashion designers and pattern makers talk about grain lines they are referring to the way that a pattern is cut out when it is laid out on a piece of fabric. Basically, fabric is woven from thread going in two different directions and it is sometimes easiest to remember that fabric is built on tiny squares of threads which criss-cross each other.
The warp thread runs up and down while the weft thread runs right to left (You can rhyme weft with left to remember which one is which). The reason why these threads are important to grain lines is that they each react in different ways. The warp thread is generally the stronger of the two and is the least likely to stretch out of shape. So for example on the straight front placket of a shirt, you don’t want it to go out of shape so if you align it with the strongest threads then it will hold it’s shape better.
The straight grain is the grain used most often in garments. The straight grain generally runs up centre front and centre back of garments and up through the centre of sleeves and pant legs. In situations where a garment is cut slightly off grain this may cause sleeves or pant legs to twist around the body. You often see this as a problem in cheap t-shirts because the fabric weave does not hold a solid grid pattern, making the fabric hard to cut correctly and causing the garment to be made up out of pieces which are off grain.
On areas such as waistbands which hold tension, you want the strongest thread to run around the body so you would cut your waistband patterns following the straight grain, ie parallel to the selvedge. The selvedge is the band of more tightly woven fabric that runs up either side of the fabric meterage. If you imagine the fabric being woven on a loom then these are the edges where the thread turns back on itself to begin weaving the next row.
In contrast to the straight grain you can also design garments which use the grain on a 45 degree angle, this is called bias cut. This effectively means that of that tiny weave of fabric you are going diagonally across the squares and making the fabric much more unstable. But while unstable sounds like a negative it can sometimes be what you need for a garment. It creates the ideal flexibility for creating bias cut dresses where you need the fabric to mould better to the body and will probably allow for less darts.
You always need to be careful of how different grain lines affect each other when they meet at seams though, as sometimes seams can stretch when cut at a strange angle or on different grains to each other and this can cause puckering.
On patterns the grain line is usually marked with a line with arrows on it, shown below in red.
When cutting out, the pattern will be laid with the grain line parallel to the selvedge.
Diagrams by The Cutting Class»