A darn that fails usually fails for one of two reasons: it was worked at the wrong tension, or it was the wrong kind of darn for the fabric. Knitted and woven cloth are built differently, they break differently, and the repair has to follow the structure rather than fight it.
Why the fabric type changes the repair
A woven fabric is made from two perpendicular thread systems, warp and weft, interlaced at right angles. Research published in Science Advances in 2017 describing textile actuators sets out the contrast clearly: the interlaced warp and weft construction of woven fabric gives a dimensionally stable structure, while knitted fabric is built from rows and columns of loops, described as courses and wales, producing a more open construction with a much greater degree of yarn mobility.
That difference explains the failure modes you meet at the mending basket. Cut a woven and the individual warp and weft ends have nothing holding them at the cut edge, so they work loose and fray. Break a single loop in a knit and the loops above and below it have lost their anchor, so the structure unravels along a wale. A woven repair therefore has to rebuild an interlaced grid and secure loose ends. A knitted repair has to reconstruct loops or, at minimum, capture the live ones before they run.
A darning egg is not optional
A stocking mushroom, darning egg or even a smooth wooden ball held inside a sock does two jobs. It holds the fabric in the shape it will actually take when worn, and it stops you pulling the darn tighter than the surrounding cloth. A darn worked flat on a table will pucker the moment the garment stretches over a heel, and a puckered darn abrades against the shoe on every step.
Keep the fabric supported but not stretched taut. If you stretch it hard over the mushroom, the darn will be too loose once the garment relaxes.
Woven darn for holes
The woven darn rebuilds missing cloth from scratch. Work a set of parallel threads across the hole, anchoring each one in sound fabric well beyond the damage, then weave a second set through them at right angles, going over and under alternately to recreate an interlacing. This works on both wovens and knits when there is an actual hole to fill.
It is an old and well documented household skill. Cooper Hewitt, Smithsonian Design Museum, holds an 1836 darning sampler and notes that young girls in eighteenth century Britain and Holland learned to mend on such samplers, filling holes with patterns that reproduced woven structures. The museum also points out the plain economic logic behind all of this: repairing cost far less than buying new.
Swiss darning for worn but intact knitting
Swiss darning, also called duplicate stitch, is for knitted fabric that has thinned but not yet holed. You trace the path of the existing stitches with a needle and matching yarn, laying a second yarn exactly over the original loop by loop. The result is a doubled fabric in that area with no change to the surface pattern.
This is the single highest value knitwear repair, because it can be done before there is a hole. Hold a jumper elbow or a sock heel up to a window. If you can see light through it, that patch is a hole in waiting, and duplicate stitch over it now takes ten minutes instead of an hour later.
Matching yarn to the garment
Match fibre first, weight second. Wool darned with cotton will behave differently in the wash, and after a few cycles the two areas shrink at different rates and the repair distorts. If the original yarn is a plied fingering weight, splitting a heavier yarn down to a comparable thickness gives a better result than using a thicker yarn at lower stitch density.
Save the offcut yarn that comes with knitwear. It is exactly the right match, and it is the only time the match will ever be free.
Repairing before it is a hole
Wear-related failure is the main reason garments leave wardrobes. A 2022 European Environment Agency briefing reports that 40 percent of all reasons consumers give for discarding clothes are linked to functional changes in the garment, including holes, worn appearance, loss of elasticity, stains and fading. Most of those are the reversible kind if you catch them early.