Socks always needed regular mending when I was a child because they were knitted from pure wool which wore away rapidly where it rubbed against the hard skin on people's feet.
Wooden darning mushroom.
Holes in socks were mended by darning. There was a special darning mushroom made of wood which was put inside the sock behind the hole to hold the darning area taut for working. Its 'stalk', with the rest of the sock around it, was held in one hand and the darning needle was worked with the other hand.
Darning needle: long with a large enough eye to take wool.
Darning needles had larger eyes than regular sewing needles so that they could take the thickness of the darning wool. They were also longer than regular needles so that they could weave in and out across a reasonably sized hole in one go.
Around the edges of the hole, the sock was always thin and weak. So the darning had to be taken some way beyond the hole into the sound area. Otherwise it would quickly pull away in wear.
Darning wool, sold in small skeins.
My mother bought special wool for darning which was sold in small skeins in the typical colours needed for darning.
Darning wool was not pre-shrunk. So it was crucial to remember that darns would shrink when the socks were washed. So darning was not merely a matter of criss-crossing and inter-weaving the wool. Little loops had to be left at each end of the toing-and-froing, as shown in the sketch.
Loops left when darning socks to allow for shrinkage of the darning wool on washing.
After the next wash, when the darning wool had shrunk, the loops would be entirely gone. If the loops were forgotten, the whole darn would shrink into a lump puckering the sock around it, making the sock uncomfortable to wear and quickly damaging the rest of it.
Often the soles and heals of socks were more darns than original sock.
These childhood recollections are of how women from working class families darned their familys' socks.