What is the J-hook on an interlocking bronze threshold?

The J-hook is the bronze strip fastened to the bottom edge of the door. When the door closes, the hook engages the matching bronze threshold and creates the interlocking seal at the bottom of the opening.

It is normally recessed into a rabbet in the door bottom so the metal is protected and positioned at the correct height.

See the replacement bronze J-hook strip. For the complete fitting sequence, see how to install an interlocking bronze threshold.