The warp must be strong to be held under high tension during the weaving process, unlike the weft which carries almost no tension. This requires the yarn used for warp ends, or individual warp threads, to be made of spun and plied fibre. Traditionally wool, linen, alpaca, and silk were used. However, improvements in spinning technology during the Industrial Revolution created cotton yarn of sufficient strength to be used in mechanized weaving. Later, artificial or man-made fibres such as nylon or rayon were employed.