A half-knot is the first step in tying a simple knot, where you cross two pieces of rope or string and loop one end through. If you've ever started to tie your shoes, you've made a half-knot: you cross the laces and pull one through the opening. But if you stop there, your shoes won't stay tied for long because a half-knot by itself tends to slip loose.

The term matters most in sailing, climbing, and other activities where reliable knots are essential. A half-knot isn't strong enough for serious work. Sailors and climbers complete the knot by adding a second half-knot on top, creating what's called a square knot or reef knot. Two half-knots tied in opposite directions hold firmly. Two half-knots tied the same way create a weak granny knot that can fail when you need it most.

Think of a half-knot as half-finished: it shows you've started the job, but haven't completed it properly yet.