The clove hitch is an easy and useful knot.
Take a length of rope, stand in front of a table and coil a loop of rope on the table. Then coil another, and then slip the second loop under the first loop. With practice you can do this very quickly.
This knot is excellent for wrists, ankles, breasts and (male subs) the ball sack. It doesn't loosen by itself unless the rope you use has no grip.
Many kinds of rope are absorbent and can transmit STD if stringent hygiene is ignored. Braided nylon rope can be soft and comfortable and is easy to clean, but can burn when it runs. Thick cotton rope is very comfortable but far too absorbent for good hygiene in group or SM play.
An excellent tie can be made from a Tee shirt. Lay the shirt on a table, and cut across just under the arms. Then cut off the hem. Then cut the remaining tube into three equal tubes. Roll each tube tightly from the edge to make what looks like a smooth rope loop. This is very flexible, but makes excellent ties, comfortable but strong enough for serious bondage play.
Use clove hitches in preference with this kind of tie. These loops can be used for a hog-tie and the great advantage is speed. You can complete the immobilisation of the sub literally in seconds, and get on with post bondage play.
Clove hitches made from Tee shirt loops can be used to 'capture' a slave by her wrist. A loop can be fed through a large body ring and then through itself for use as a leash. There are endless possibilities. A single loop can be used to make two clove hitches to secure a pair of slaves by their wrists, back to back, or a single slave's wrists with a bit of freedom for sexual play. You can also use these loops to secure a slave to a chair or a table, or stretch her out on a bed.