The vast majority of the buildings in the historic parts of Capitol Hill are built with “frog-less” bricks.
It is said that the slop molds which were once used to create bricks in the mold making process, with an indentation set in, look like a frog with its legs at its side and its torso in the middle of the box. There may be no documentation to confirm the original use invention of this method of molding bricks, however this explanation of the etymology seems plausible.
The frog is an indented cast, molded or formed into the bottom of a masonry unit. A brick created with a frog has several advantages. It is a more stable method of setting the brick into the mortar bed. That three-dimensional shape that’s left by the frog adds stability because it provides a key for the mortar to fill and lock the brick in laterally. The presence of the frog also allows a brick of the same size to be shipped or transported and used in construction at a lower weight, which saves fuel cost and reduces labor for installation and construction.
Later, after frogs were first used in brick production, each brick manufacturer began putting their name into the frog. This gave the brick manufacturer an opportunity to do a little bit of marketing and the letters in the name also added yet a more three-dimensional surface to which the mortar could embed.