Primarily this is what stops the lace from coming undone.In effect, the lace is narrower inside the knot than it is on the loose end, and the loose end cannot make itself smaller and slide though the knot.The stiff section at each end of the shoelace, which both keeps the twine from unraveling and also makes it easier to hold the lace and feed it through the eyelets, is called an aglet, also spelled aiglet.
Modern shoelaces often incorporate various synthetic fibers, which are generally more slippery and thus more prone to coming undone than those made from traditional fibers.
On the other hand, smooth synthetic shoelaces generally have a less rough appearance, suffer less wear from friction, and are less susceptible to rotting from moisture.
Generally, a flat tubular lace will stay tied more easily than a round lace with a core because the flat lace can be more crimped within the knot.
Most laces, however, are round and have core of cotton yarn, especially boot laces.
Shoelaces can be coated, either in the factory or with aftermarket products, to increase friction and help them stay tied.