Which knot is used to secure two ropes of different diameters together?

The Double Fisherman's knot is specifically designed to join two ropes of different diameters securely. This knot is particularly effective for this purpose because of its ability to grip and hold tightly under tension. When tying the Double Fisherman's knot, each rope wraps around the other and creates a locking mechanism that enhances its strength, making it less likely to slip compared to other knots that may not hold as effectively when different sizes of rope are used.

While the Square knot is used to tie two ropes of similar diameters but can slip when used with different diameters, the Clove hitch is mainly utilized for fastening a rope to a post or a ring and does not secure two ropes together. The Figure-eight knot is primarily designed for creating a stopper or tie-off at the end of a rope, again not serving the purpose of joining two differing diameters. Therefore, the Double Fisherman's knot is the most suitable choice for this application.