The needle loop is the fundamental building block of knitted structure. In weft knitting, when fabric tension is balanced and adequate take-away tension is maintained during the knitting process, the loop takes the form of an upright noose created inside the needle hook. It is typically composed of a head (H) and two side limbs or legs (L). At the base of each leg is a foot (F), which interlocks through the head of the loop produced in the previous knitting cycle, usually by the same needle. The yarn path continues from the foot of one loop into the foot and leg of the next loop formed by that needle.
If the loop is the first loop knitted on a particular needle, its feet and legs are not yet confined by adjacent loops, allowing it to open out and resemble a tuck loop. On flat knitting machines equipped with a presser but without take-down tension, loops tend to become more rounded and may also incline as a result of the presser’s traversing movement.
In general weft knitting, loop feet remain mostly open because yarn is continuously supplied from one direction, except at the selvedges of straight knitting machines. Closed loops are less common. Historically, they have occasionally been produced on bearded needle sinkerwheel machines by twisting a loop during transfer to another needle, or by using a twizzle beard that closes against the back of the needle, causing the loop to twist over itself at the casting-off point.