The HL is simply a bit of a sturdier option to the simple H hinge. The extra key is an extra supporting arm that attaches to the front side of the door you are fastening, which is an extra L shape that attaches to the H.
This is a great option if you’re looking for extra support for a heavy wooden door. There’s also the option of adding hinges on the bottom hinges as well, where the bottom front hinge will be G shape that attaches to the H shape.
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