Which knot must always be used in conjunction with other knots?

The half hitch is a versatile knot primarily used for securing rope to a post or object, but it is generally not strong enough to be used as a standalone fastening. Instead, it is typically used in conjunction with other knots to create a more secure configuration. For example, when tying off a load or creating a secure anchor point, a half hitch can be used in combination with a more robust knot like the bowline or clove hitch. This combination enhances the overall stability and security of the knot system, ensuring that the load or the object being secured remains in place.

In contrast, other knots such as the square knot, clove hitch, or double fisherman’s knot can often be used independently to achieve their specific purposes without requiring additional knots for enhanced security. Therefore, the half hitch's dependence on other knots for maximum effectiveness distinguishes it in this context.