The basic function of selvedge is to lock the outside threads of a piece of cloth and so prevent fraying.
It is easy to achieved if the weft yarn is inserted continuously from one package, such as a pirn on a conventional shuttle loom, because, each time the package carrying shuttle reverse its direction, a hairpin of weft yarn will form round the last thread in the cloth provided that this end changes its position between picks.
Requirement for selvedge:
Strong enough to withstand the strains during finishing process.
The selvedge should have a neat and uniform appearance.
Uniform to allow garment manufacture to line-up one edge of the fabric; layer upon layer so that many layers can be cut simultaneously and accurately.
Uniformity of appearance may be a problem if the outside ends in the fabric do not change position every pick.
Happens in twill and satin-type weaves.
If the appearance of the selvedge is unacceptable when one of these weaves forms the ground structure, then it will necessary to use some special technique of threading up or to introduce a plain-weave selvedge for a width of up to 1 cm at each side of the fabric.
Alternatively, it may be quite satisfactory to make only the outside end in the fabric change position after each pick and it is known as a catch cord or catch end.
The most successful in preventing fraying and providing strength but as successive picks are beaten-up the weft tails are made to point up and down alternately by the crossing end so that the fringe does not appear.