Straight Grain or Cross Grain Binding
The most common binding is cross grain binding. Strips are cut across the width of fabric and then are pieced on the diagonal. The seams are pressed open. Straight grain binding is cut on the lengthwise grain and strips are also pieced on the diagonal. The slight advantage of cutting binding on the cross grain is the binding has a slight "give" or stretch if there is a necessity to ease.
For cross grain binding if you cut 2 1/2" strips you will have 40 usable inches of fabric. When you piece on the diagonal, you will loose almost three inches of each strip due to the seam and trimming off the waste triangle and selvage. You would need to cut enough strips to accommodate the seaming and waste. Cross grain binding works very well for square or rectangular quilts.
Straight grain binding is also cut in strips but the length will vary depending on the length of usable yardage. Again you will loose in the seaming the same as for the cross grain strips. Straight grain binding should be used if the edges of your quilt are not on the straight grain or cross grain. The straight binding will help keep the quilt square. This binding might be best for a wall quilt or art quilt.
If you have a round quilt or scalloped edge quilt you will need to use bias binding. The bias binding has "give" or stretches on the bias and it is able to curve around a circular portion on the outside of your quilt.
Many special treatment bindings are also cut on the bias. If you have a striped fabric, or any one way design, cutting binding on the bias adds a diagonal interest to the binding edge of your quilt.
Bias binding will wear longer than straight binding so in a bed quilt that is getting lots of use bias might be the better choice. The straight binding will have only one or two threads actually on the edge of the quilt. These few threads in the fabric right on the edge of the quilt will wear faster. Bias binding has stretch and flow and moves at the edge and won't wear as quickly.
Many quilters think that bias binding takes more fabric than straight binding. This is really not true -- if you calculate the square inches of fabric it takes to make straight or bias binding you will find the amount of fabric needed is about the same.
Straight binding is usually cut from straight strips the width or length of the fabric and bias binding is usually cut from a square. It only appears to be more yardage when cutting bias from a square.
Flat Binding and French Fold Binding
French fold binding differs from flat binding. Flat binding is just that -- flat. It is not folded before applying to the quilt. Flat binding is usually cut narrow than French binding. Flat binding is stitched right sides together on the top of the quilt using a quarter inch seam. Then turned to the back and the quarter inch raw edge is turned under as it is hand stitched. If you are short on fabric flat binding might be an option.
French fold binding is cut wider -- usually 2" to 2 1/2" is standard. Then fold the pieced strips in half lengthwise with wrong sides together and press. Then it is stitched to the quilt -- matching raw edges of binding with the raw edge of the quilt. Use a quarter inch seam. After stitching, turn binding to the back of the quilt and hand stitch. The folded edge is already finished and ready to stitch to the back of the quilt with no additional turning.
Using French fold binding -- either straight or bias -- will assure that even if the edge of the binding is wearing the inner parts of the quilt won't show because there is a second layer of binding behind the top layer. You won't have to replace French fold binding as often as the edge of the batting won't show.
Remember -- if you are making a planned quilt and know what fabric you will be using for binding, why not cut the square for bias binding or strips for straight binding at the same time you are cutting your strips and patches for piecing. Then label it by pinning the name of the quilt on a small scrap of paper to the binding fabric or strips. Then when the quilt is finished you will have your binding ready and won't have to hunt in your stash -- or worse have used all the fabric in another project.
To stitch binding to your quilt whether it is bias or straight it is helpful to use a walking or even feed foot. Your quilt is already three layers and they are not always quilted right up to the edge so you are still dealing with three layers. Then you have the two layers of the French fold bias or straight binding.
Not a long time ago, I received a somewhat silly question that I would like to address publicly.
Is there an advantage to measuring it as 6/8" rather than the reduced fraction of 3/4"? Are people more accurate looking (and thinking if it) in smaller increments than larger ones? I ask this sincerely, not to be a smarty pants.
There is no right or wrong answer here. There is not an advantage to measure 6/8" instead of 3/4" as it is exactly the same measurement when cutting fabric. Yet if it is easier for you to think in eighths and cut in eighths rather than reducing it to quarters when possible -- it is perfectly OK for you to think and cut your fabric in eighths. As long as you cut your pieces accurately it doesn't make any difference if you think in fourths or eights -- just so you count correctly on your ruler.
For accurate cutting with acrylic rulers and tools be sure the entire line printed on the ruler covers the fabric. Don't cut your fabric with the line to the right or left because that will cause cutting inaccuracy. The rulers and tools are calibrated to include the printed line when cutting.