Sheet Bend

Join two ropes by wrapping one end around a bight in the other.

Put the bight in the thicker rope and keep both short ends on the same side.

Step-by-step

Form a bight
Fold the thicker rope. Bring the thinner rope toward the opening.
Enter the opening
Pass the thinner end through the bight from behind.
Pass the right leg
Bring the end forward and up beside the bight's short end.
Wrap both legs
Carry the end behind both legs and around the left side.
Tuck under the entry
Bring the end across the front, then under its own incoming strand.
Extend the tail
Leave both rope tails on the same side of the knot.
Dress and tighten
Pull the standing parts apart and guide the turns together. Keep the tails clear.
Inspect the knot
Check one turn around both bight legs with the tail tucked under its own entry.