Final answer:
The square knot or reef knot is commonly used to tie two lines together. It is a simple and versatile knot that is used in various applications.
Step-by-step explanation:
The knot that is commonly used to tie two lines together is called the square knot or the reef knot. This knot is known for its simplicity and versatility, and it is commonly used in various applications such as camping, fishing, and crafts. To tie a square knot, you cross one end of the line over the other, then pass it underneath and through the loop created, and finally tighten the knot by pulling both ends in opposite directions.