Page 90 - Berndt Sundsten "My First Book of Knots: A Beginner's Picture Guide"

The slip knot is a knot that can be undone very quickly. The illustration
shows a slip knot together with two round turns. The slip knot of slip knots is
the highwayman’s hitch, shown on page 12. The slip knot is a good knot if,
for example, you want to tow a boat. The knot is easy to undo if a situation
arises and you want to quickly release the boat being towed.