Double-ridged waveguide can provide more bandwidth than "normal" rectangular waveguide. Double-ridged waveguide is similar to finline.

A pair of ridges protrude into the center of the waveguide, parallel to the short wall. This is where the E-field is maximum. By bringing ground down the ridges, the E-field is further increased.

That was a lame explanation... while we're waiting for more content on double-ridged waveguide, let's step back and look at its cousin.