The sheet bend is the knot used when you need to join the ends of 2 lines of unequal size or type. It is also used by fishermen to make and repair nets. It can also be used to make big game nets or hammocks. There are countless ways that knowing how to tie a sheet bend would come in very handy in a survival situation.
Create a bight in one of the lines, usually the largest line.
Poke the bitter end of the other line up through the bight.
Make a single round turn about both the standing part and bitter end side of the bight.
Slip the bitter end you are working with under the loop that comes up through the bight but do not poke it back through the bight.
Draw it all snug without letting it tumble.
Once this knot is snug, it will stand whatever stress the weakest line will safely bear. If it tumbles before it is drawn tight, you have nothing and it will fall apart. Remember, any knot is exactly right or it is just wrong. Take care when tying any knot as the wrong twist or turn has the potential to end with very serious consequences.
When joining two pieces of line, whether of equal or unequal size or type, the fastest, easiest, and safest way to join them is with the sheet bend.