The most common knot in everyday life, relatively secure, sometimes difficult to untie. However, it may fall undone on artificial threads or threads of different diameter if the running ends are not fixed. The Reef knot is easy to make a mistake, so one should tie it carefully.
ABoK #74, #75, #460, #1204, #1402, #2096, #2573, #2574, #2577, #2580
Fold the rope to form a loop with the running end upside.
Thread the 2nd rope through the loop under the 1st standing part.
Pass the 2nd running end over both ends of the 1st rope.
Pass the 2nd running end under the 1st and through the loop.
Tighten the knot.
The Reef knot is complete.