Name: reef knot
Also known as: square knot
Useful for: joining two ends of rope
Knot style: jamming, binding
The reef knot is one of the simplest and oldest knots. It is very versatile and probably one of the first knots most people will learn. It is certainly the first knot I remember being taught in Scouting.
To make this knot you take two ends of rope and go "over-under then under-over"*, as I was taught, before pulling taught. The knot tightens easily and it is a jamming knot so it only gets tighter as you pull on it. However, it is not a good knot for binding in actuality because the ends of the rope need to be matched or it can slip, defeating the point of the knot. If you pull one end away from the knot it can also rotate (known as capsizing) which again, defeats the point of the knot.
It is an excellent knot for teaching some basic knot skills and for quickly tying two things together. I can't stress enough that it should not be used for anything where a life may be on the line for the limitations mentioned above.
Tom out!
*Also formulated as "right over left then left over right".
Attribution
The Reef Knot used under CC by SA 3.0