A grain of fabric refers to the direction of the threads in the fabric. Fabrics have three grains: lengthwise grain (parallel to the selvage edge), crosswise grain (perpendicular to the selvage edge), and bias grain (45-degree angle to the lengthwise or crosswise grain). Each grain behaves differently when fabric is cut or manipulated.
The way the fabric is woven gives it different properties in different directions. When you are cutting a pattern you want to get all the pieces on the same "grain", i.e. a piece that will be vertical on your body should not be cut diagonally on the peace of fabric (unless you cut all the pieces diagonally or on the bias). The grain of the fabric is the natural direction of the fabric, usually up and down along the length of the fabric.
bias
BIAS
BIAS
Straight line ending in arrowheads, means "place on straight grain of fabric."
The way the fabric is woven gives it different properties in different directions. When you are cutting a pattern you want to get all the pieces on the same "grain", i.e. a piece that will be vertical on your body should not be cut diagonally on the peace of fabric (unless you cut all the pieces diagonally or on the bias). The grain of the fabric is the natural direction of the fabric. The selvage is the edge of the fabric. When you buy a piece of fabric from the roll, you get two cut edges that fray and two factory edges that look different from the rest of the fabric and don't fray. These edges are the selvage. If the grain of the fabric runs from selvage to selvage that means the natural direction of the fabric (that you should use to line up your pattern pieces) runs straight across the fabric from factory edge to factory edge.
The fabric for garments is cut in a way that will create the best lay when constructed. Fabric tends to stretch along the grain, which is why abutting seams are typically both cut along the grain.
Cross grain means running across the regular grain of a piece of wood.Cross grain or crosswise grain in fabrics means that the fabric grain has more stretch than lengthwise but less than bias.
Bias
a line going diagonally across the grain of fabric
For sewing pattern layout, folds are made along the grain line. Lengthwise center fold would be folding the fabric with selvages together. Crosswise centerfold is folding the fabric at the crosswise grain, that is along the weft threads from selvage to selvage.
That is the bias. When you have a pattern that states "cut on bias" the pattern piece would be placed in a fastion that would be a "diagonal line against fabric grain". Pattern pieces cut in this fashion are sometimes more difficult to sew because when fabric is cut on the bias it gives the cut edge more of a stretch.