The sheet bend is the most important knot for joining two rope ends, especially if the ropes are of different sizes. Sailors named it in the days of sailing ships when they would “bend” (tie) the “sheets” (ropes attached to the clew of the sail).
Begin with a bight in the larger rope. Then weave the end of the smaller rope through the eye, around the bight, and back under its self. Snug it carefully before applying any strain to the knot.