Twill weave, also known as 'Drill' is one of the key textile weave structures along with plain and satin weave. It is made by interlacing the warp and weft yarns in various ratio forms, but must incorporate a 'step' in the pattern to form it's defining diagonal rib. This diagram for example shows a 3:1 weave (over three warps, under one weft, with a step to form the diagonal pattern). The angle of the twill is generally 45°, but can range between 15-75° depending on the weave ratio. The diagonal rib can be woven in both north-west to south-east, or north-east to south-west directions, but it is easier to adopt the S and Z-twist terminology mentioned in the 'Fibre to Form' blog, and describe the angle as S or Z-twill. This diagram illustrates an S-twill weave.
Twill weave is used to make hardwearing fabrics such as denim and gabardine. The diagonal rib helps to give it firm but smooth drape, as well as a discrete texture that helps to disguise marks or stains on the fabric should they appear.