The red one is a pipe wrench The middle one is what many people call a crescent wrench. The crescent wrench was so-called because it was originally manufactured by Crescent Tool Company. That company makes wrenches in a variety of other designs, but the name "crescent wrench" has come to refer to the design pictured, and it is still called a crescent wrench when it is made by other companies. The bottom one is a monkey wrench. The monkey wrench is named for Charles Moncky, who invented it around 1858. Another name for the pipe wrench is Stillson wrench: Daniel C. Stillson got a patent for it on September 13, 1870.
I've added the labels "A", "B", and "P" to help in describing the operation of the pipe wrench. The piece that has jaw "A" pivots a few degrees around point "P". As the handle is moved down, due to the pivoting action jaw "B" is pushed against the pipe that is between the two jaws. The harder you push down on the handle, the tighter the jaws squeeze the pipe. If the handle is moved in the opposite direction (counter clockwise in the picture), jaw "B" pulls away from the pipe, thus loosening the grip of the jaws on the pipe. That's why a pipe wrench works in only one angular direction.
The jaws of a pipe wrench have sharp teeth that bite into the pipe to prevent the slipping that would occur even when the jaws squeeze tightly on the pipe.
The pictures at right illustrate the pivoting action that produces great clamping leverage with a
pipe wrench. The top picture shows the jaws with no force applied. The bottom picture shows what
happens when the jaws are forced open. I've done the forcing with my finger, but adjusting the
opening of the jaws to a little less than the diameter of the pipe and placing the jaws
over a pipe produces the same effect.
