Homburg hat

A homburg is a semi-formal hat of fur felt, characterized by a single dent running down the centre of the crown (called a "gutter crown"), a wide silk grosgrain hatband ribbon, a flat brim shaped in a "pencil curl", and a ribbon-bound trim about the edge of the brim.

[1] Although the homburg is traditionally associated with semi-formal wear — namely, the dinner jacket (tuxedo) and black lounge suit (stroller) — it has been extensively applied also to informal attire.

[5] At his 1953 inauguration, Dwight D. Eisenhower broke with tradition by wearing a black homburg instead of a top hat.

[8] Some Orthodox Jewish rabbis wear black homburgs to the rekel, though this practice is also in decline.

It was sometimes jocularly referred to as a "hamburger", notably by actor Edward Brophy in the 1958 film The Last Hurrah.