What is the name of the knot used to temporarily combine two ropes of different sizes?

The Sheet Bend is the appropriate choice for temporarily combining two ropes of different sizes. This knot is designed specifically for joining two ropes that may not be of the same diameter, making it particularly useful in a variety of scenarios in rigging and marine applications. The way the Sheet Bend works allows it to grip the thinner rope snugly, even when it is tied to a thicker rope, which helps to prevent slippage.

In contrast, the Bowline creates a fixed loop at the end of a rope and is not suitable for combining two separate lines. The Square knot, while effective for tying two ropes of similar size together, can slip if the ropes are of different diameters or materials. The Figure Eight knot is primarily used as a stopper knot or to create a secure loop and is not designed for joining two ropes together.

Thus, the Sheet Bend stands out as the best choice for this purpose, making it a fundamental knot to know for anyone involved in rigging or other rope-handling activities.