What distinguishes first class levers from both second and third class levers?

The fulcrum is between the load and effort distinguishes first class levers from both second and third class levers.

Explanation:

Lever is a bar that can rotate around a fulcrum. The levers can be divided into three classes: First Class Levers (Interlocking Levers), Second Class Levers (Inter-Resistant Levers) and Third Class Levers (Interpotent Levers). In the first-class levers, the fulcrum is between the point of application of the force of action and that of the resistance force.

The fulcrum is between load and effort distinguishes first class levers from second and third class levers.