Quilting is not just the simple pushing of needles and threads in and out of fabrics and come up with a beautiful craft appreciated for years. Like in any other art form, there are pieces of information a quilter has to master to be able to improve his craft.
In quilting, knowing and understand fabric grains is a plus for quilt-makers who want to excel.
Straight grain
Simply put, this is the way threads are woven in place in the fabric. This is important when quilters want to cut their quilting blocks accurately and with precision.
Ignorance in fabric grains can result in great waste of time, effort and materials.
Long threads are called warps. These are the ones that are stretched and secured on the loom. In effect, these become the fabrics lengthwise grain, the continuous fabric yardage as it comes off the bolt.
The weft is the thread that is woven into the warps, perpendicular to the whole length.
This is the crosswise grain of the fabric.
The lengthwise grain and the crosswise grain are both called straight grain, or straight-of-grain. The selvage is the bound edge in the outermost lengthwise grain, formed when the weft thread change direction in the weaving process.
Fabric pieces cut along straight grain (with edges parallel to either warp or weft) are less likely to stretch out of shape. The interwoven threads give it extra support.
However, cuts along the lengthwise grain stretches less than their crosswise counterparts. This is because warp threads were firmly attached to the loom enhancing their strength.
Fabric Bias
A bias is a 45-degree angle cut in the fabric. (In quilting, any cut that is not parallel to the straight grain is called a bias cut.)
Bias cuts are stretchy and this can work for or against you. There are no threads to stabilize the fabric in a bias.
However, bias cuts have several uses. One, they are easy to apply as binding to a quilt with curved edges. Tubes can be made from them for shapes in appliqus, for instance.
They are also easier to be turned under for other curved shapes.
Bias edges can stretch out of shape and makes it difficult to sew accurately. Triangles have one bias edge and there is need to determine the best place for it because it is not stable. (Best is sewing them in the interior parts and with a straight grain piece for stability.)
Using straight grain cuts
Squares and rectangles are cut along straight grains to minimize stretching during handling. They are good in making borders and sashes. Moreover, they stabilize the outer edges of blocks and quilt tops.
Knowing and being accustomed to these different fabric grains would give you quite an idea on where and when to use any on your project. It can even make you quite an expert in identifying fabrics without the usual selvages or other marks.
At a certain stage in your quilting life, you can do your own fabric grain experiments. It does not take so much to know the best possible ways to achieve what you want in your quilt masterpiece.