The grainline describes the direction of yarns or knitted columns in fabric. On a pattern piece, it is usually shown as a long arrow that tells you how to orient the template. Direction affects stretch, drape, and whether the finished item keeps its intended shape.
Lengthwise, crosswise, and bias grain
In woven fabric, warp yarns run lengthwise and parallel to the selvages. This is the lengthwise or straight grain used for most pattern placement. Weft yarns run across the width from selvage to selvage. The true bias lies diagonally, about 45 degrees to both; woven fabric generally has more give on this direction.
Knits do not have warp and weft in the same constructional sense. They still have a lengthwise direction, visible in the vertical wales of many knits, as well as a direction of greatest stretch that the pattern may specify.
Aligning a grainline arrow
For a straight-grain piece, position the arrow parallel to the selvage. Measure from one end of the arrow to the selvage and then from the other end. The measurements should match before the pattern is secured with weights or pins.
A line with bent arrowheads often marks an edge to place on the fabric fold. A pattern intentionally designed for bias cutting follows its marked diagonal orientation instead of the lengthwise grain.
What happens when fabric is cut off grain?
An unintended angle can make long pieces twist, side seams rotate, or hems hang unevenly. Checks and stripes may also appear skewed. The effect is especially noticeable in trousers, skirts, and long sleeves where a small deviation continues over a greater distance.
Some designs deliberately use the bias to create fluid drape or make bias binding. In that case the altered behavior is part of the design. Rotating pieces only to save fabric, however, changes how they stretch and hang and may compromise fit.
A cutting layout accounts for these directions, but the marking on each piece remains the final guide when creating a custom arrangement.