Finger Joint Router Bit is the most common and ideal for wood joining by end-to-end and edge-to-edge.  The router works with one workpiece face up, the other face down. When the bit height is correct, the two pieces should slide together with their faces perfectly flush. 

Finger Joint Router Bit assembly includes five 3 carbide tipped wing finger cutters, a ball bearing guide, a 1/2" shank, shims, spacers, and washers. With its multiple interlocking fingers, this bit creates maximum gluing surface area for superior edge joints and splice joints. The fingers have a wedged shape, which when clamped together, helps to close up the joint without any gaps. 
Note: For best results run at full speed in a 1-1/2 horsepower table-mounted router.