bias
BIAS
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.
an oblique or diagonal line of direction, esp. across a woven fabric.
a line going diagonally across the grain of fabric
Straight line ending in arrowheads, means "place on straight grain of fabric."
No, it does not. Take a sheet of A4 paper and fold it across the diagonal, and you will see that the two halfs do not line up.
Three vertical lines, |, with a diagonal line across them.
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.
right triangles and
Fabric which is cut 'on the bias' is neither cut across the weave nor along it, but at a diagonal between them - imagine the diagonal of a square and this is the line that a bias cut follows. Farbic cut in this way tends to drape around the body well and 'clings' far more than a standard cut.
It is a diagonal