From the online Glossary:

HARPIN - A sheer sawn to shape as opposed to being bent to shape around the frames. The harpin may be only a portion of the sheer (usually the forward section) or the complete sheer. Glen-L supplies a pattern or dimensions when a harpin is called for.

Designs with a harpin cannot easily be extended or shortened. If a design has a harpin, it will be listed in the description of the plans and patterns on the design page.