2-way: One inlet and one outlet, and are used to permit and shut off
fluid flow. The two types of operations are "Normally Closed" and
3-way: These valves have three pipe connections and two orifices. When
one orifice is open, the other is closed and vice versa. They're commonly
used to alternately apply pressure to an exhaust pressure from a valve
actuator or a single acting cylinder. These valves can be normally closed,
normally open or universal.
4-way: These valves have four or five pipe connections, commonly
called ports. One pressure inlet, two cylinder ports providing pressure to
the double acting cylinder or actuator and one or two outlets to exhaust
pressure from the cylinders. They have three types of construction, single
solenoid, dual solenoid or single air operator.