Follow the threads
In woven fabric, the lengthwise threads, or warp, run parallel to the selvages. The crosswise threads, or weft, run across the width. True bias runs at 45 degrees to those directions and has more give.
A pattern’s grainline marking tells you how to align the piece with the fabric. Follow that marking and the cutting instructions, including any one-way layout needed for a directional print or nap.
A piece that fits may still face the wrong way
For example, turning a rectangle 90 degrees changes which dimension follows the lengthwise threads. It also turns the print. Check both before choosing a layout to save fabric.
The backing calculator compares layouts that allow panels to rotate. If your fabric or quilting instructions require a particular direction, check that the suggested plan is suitable before cutting.