A compound machine is made by combining two or more simple machines. Scissors is a compound machine because it's a combination of a wedge and a lever, which are both simple machines. The cutting edge of scissors is a wedge and the cutting force comes from a lever.
In a compound machine, one type of simple machine transfers its input force to another simple machine connected to it. The six basic simple machines types that can be found in compound machines are grouped into two families: the lever family and the inclined plane family. The three simple machines in the lever family are the lever, the pulley and the wheel and axle. The inclined plane, the wedge and the screw are the three simple machines in the inclined plane family.
The simple machines in the lever family perform work by an input of force applied at some distance from a fulcrum, which is a pivoting point. An application of force at one side of the fulcrum results in a transfer of force to another part of the machine. A see-saw has the fulcrum located at the middle of a rigid arm, but the fulcrum could also be at the opposite end, as in the case of a wheelbarrow. In a pulley, the fulcrum is the wheel that the rope or chain rotates around.
The inclined plane family of simple machines makes use of a slope to allow something to move up or down, or in the case of a wedge, to spread things apart. A screw holds things together through an inclined plane wrapped around a central shaft. Simple machines from both the inclined plane family and the lever family can be found working together in a modern compound machine such as a bulldozer.