This section is from the book "A Sewing Course For Teachers", by Mary Schenck Woolman. Also available from Amazon: A sewing course for teachers.
A bias cut in cloth is a slanting or diagonal severing of the material. Both warp and woof threads will be cut. (See Fig. 15.) It may vary with the requirements of the garment. A true or perfect bias, however, does not vary. It is always the diagonal of the square. It equally severs both warp and woof threads. (Fig. 16.)

Fig. 15. - Garment Bias. (Gore.)

Fig. 16. - True Bias.
 
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