What is a knuckle boom crane?

A knuckle boom crane is also known as a picker crane, loader crane and articulating crane. 
 
It resembles a traditional straight boom crane but it actually has two booms that articulate at the knuckle in the middle and resembles the action of the human finger.
 
Knuckle boom cranes are known for their ability to maneuver in tight spaces and their compact storage size. It has the ability to lift heavy objects and place them inside a building through small spaces, like a doorway.
&