Quilters cotton has a grain because it is a woven fabric, made on a loom. How the threads are woven on the loom determines the crosswise grain and lengthwise grain.
Often, instructions simply state to cut "on the straight of grain." Both crosswise and lengthwise are considered 'straight of grain'.
How to find the grain in fabric purchased off a bolt —
Fabric with the selvages intact make finding the grain straightforward. As the drawing above shows, the crosswise grain runs from selvage to selvage — the width of fabric (WOF). Another term often used is 'usable width of fabric', that means the width of your fabric minus its selvedges. Lengthwise grain runs along the same direction as the selvages — the length of fabric.
How to find the grain in a pre-cut or scrap —
Pre-cuts and scraps don't have selvage edges to help you identify the fabric grain. Pulling on the fabric straight of grain will tell you what you need to know!
Here's how to test —
Grab the fabric, one side in each hand, and pull your hands apart.
What do you feel?
The control QuiltSandwich's Yardage Calculator gives you over its cutting diagrams helps you use fabric grain to your advantage.
Fabric grain matters the most with rectangles, borders and binding —
Rectangle shaped pieces: Use grain to make assembly easier
For both 'Rectangle or Square' and 'Half Rectangle Triangle', QuiltSandwich gives you the choice of 'Find Best' where it chooses how to layout pieces to use the least fabric or 'Force Layout'. Putting the piece Length on the lengthwise grain will make it easier to assemble strip sets that lay flat. If you are using a die cutting, setting 'Force Layout' allows you to cut strips that will feed into the die cutter as recommended.
Border and Bindings: Choose the grain to fit your project
QuiltSandwich creates cutting diagrams for Horizontal, Vertical, Mitered Borders and Straight Binding. QuiltSandwich lets you set Binding and Borders to either Crosswise or Lengthwise grain — because sometimes you need options!
Backing and Batting pieces are so big they'll run the length of fabric, so fabric grain is not an issue.
Often QuiltSandwich doesn't offer you a grain setting for these pieces, however, if your quilt is larger than a single piece of fabric, the backing and batting will need to be pieced. If don't specify fabric grain, QuiltSandwich run the calculations both ways and suggest the direction that will use the least yardage. Of course if you have a preference, QuiltSandwich lets you choose horizontal or vertical assembly.