Typically the protruding joint, also know as a rule joint, is mortised into underside of table top. This mortising provides a flush surface enabling leaf to swing clear.
Drop leaf table hinges have two unique characteristics: a rule joint and two hinge leaves of unequal lengths.
The rule joint differentiates this hinge from other hinges as it protrudes on only one side of hinge. The hinge joint is mortised into the underside of the table top providing a flush surface enabling leaf to swing clear.
The hinge leaves are intentionally two lengths. The shorter hinge leaf is mortised into the underside of tabletop. The longer hinge leaf is mortised into the underside of table leaf.
Rule joint hinges are available in steel and extruded brass.
Many old tables used simple steel drop leaf table hinges.