A type of
ladder where two sections of ladder are joined together with two j-shaped clamps and a pair of swinging base
cams, which rests on the feet of the other section of ladder.
The reason they are called extension ladders is that you can extend them. Humans are not good at controlling objects longer than three meters (ten feet) by themselves, and 10 meter objects do not fit well on top of a service
van.
The lower section has a set of swinging feet that allow for a secure perch on the ground. The cam swings aside when the ladder is pushed up, but has a cup shape on the bottom, so when released lowers into place. There are pair of swinging guards on the cam, so the base won't engage when the ladder is being lowerd.. To raise the ladder you simply push (or pull up using an attached rope and pulley) the ladder up until the desired height is reached, then lower the cam cup onto the appropriate rung of the lower ladder. To lower the ladder you lift up again until the guards swing in place and then lower.
Extension ladders are meant to be supported by an existing object they rest against. Like a wall or beam. The rule for safe use is that the feet of the ladder should be located one foot away from the object for every four feet in height. That means if you want to take the ladder up a wall twenty feet then the ladder's base should rest five feet from the wall.
If an extension ladder is to be used
OSHA regulations require that the top of the ladder be
tied off to something solid and extend at least one meter above a roof deck if the ladder is being used for egress. This is so a person climbing on the ladder has something to grab. Tying off is required so the ladder won't slip sideways, which could
hurt.
Extension ladders can be quite long, and I have used one rated at 44 feet. Such ladders are very large, heavy and require two people to move. Extension ladders can be nerve wracking as they flex a great deal. Even a new
fiberglass ladder flexes enough to make most people uncomfortable. But they will get you places no other ladder will go, which is why they are so common.