Temples, templates, stretchers, Breithalter — are all names for an ancient weaving tool conceived to stretch the fabric to full reeded width at the fell line for a more effective beat.
Using a temple will stretch the web and allow the dents in the reed to beat squarely with more force than if the fabric has drawn in, even just a little. This leads to fabric with a square grain (warp 90° to the weft) —
smileys where the fell line is curved or wavy will disappear, the temple will allow a slight tug as the weft turns — meaning fewer loops at the edge.
Using a temple will not guarantee good selvages, though it should make producing nice ones easier. It will NOT keep fabric from drawing in due to normal weaving takeup — what the weaver is striving for is an even width behind the temple.
Temples come in many forms and have been used for many centuries. They are not something that indicates cheating or lack of weaving skills, but are a proven tool to make a better quality cloth. Actually, they are still used in industrial settings in spite of all the technological advances since the first handlooms.
A makeshift temple can easily be made from items at hand. All that is necessary is strong yarn, a pair of hooks (bent paperclips will do) and a pair of weights (nuts, film canisters full of coins of fishing weights).
The hooks are then placed into the cloth at the selvage and stretched out over the sides of the loom. If your loom has high boards on the sides, this will do. If the boards are lower running support cords from breast beam to back beam gives proper support.
Simple Temple to Make — Woolgatherers