Missing Extension Line on Drawing

When make a dimension by selecting an edge I get 2 witness lines with arrows and extension lines to the geometry.

When I make a dimension between 2 edges I get 2 witness lines, but only 1 has an arrow and only 1 has an extension line. What setting have I accidentally messed up?

I just figured it out (that always seems to happen right after I post!).  When you go to place a dimension there is a "Half Dimension" option that appears in the RMB menu.  Now the question is, where and how would you use a half dimension?