Selvage (US) or selvedge (UK) is the technical term for the narrow and tightly woven self-finished edges that function as the natural endings on each side of fabric woven on an old school shuttle loom and prevent it from fraying or unraveling.
In denim the selvage is usually white and it often has a coloured thread in the middle, which wass originally added to help manufacturers recognise the different qualities that they were producing for different clients.
The Cone Mills White Oak factory in North Carolina that started supplying fabric for the Levi’s 501 jeans in 1915, and around 1927 they incorporated a red thread in the selvage of the extra durable XX-fabric to make is easier to distinguish it from other qualities.
Today, the red thread has become synonymous with selvage fabric from any brand which may be partly due to the dominance of Levi’s in the market the first half of the last century.
However, it is still possible to find vintage jeans from that period without the red thread in the selvage, Lee jeans originally used plain white selvage denim yet sometimes you’ll find jeans with yellow thread in the selvage. Wrangler used a green thread to distinguish their fabric, however, on most traditional Wrangler jeans the outerseam is double felled (like the innerseam) compared to the busted seam that will reveal the selvage.
Selvage fabrics are woven on narrow 28-30-inch shuttle looms that throughout the 70s and 80s were replaced by more effective and wider 58-60-inch projectile looms. On these modern looms the weft (the horizontal thread) is cut at each end, which is called weft insertion. This creates frayed ends that are cut before the fabric is sewn into a pair of jeans.
On old shuttle looms the selvage is created by shooting a coil lead back and forth during weaving without cutting the thread. This creates a tightly woven and durable edge, which without additional processing may be included in the finished product.