The Groove is the plowed channel at the bottom of a type. Its function is a negative one: to remove the objectionable roughness at the bottom of the type where the Jet is broken off. In hand type casting and in pivotal type casting machines, the type is delivered with the Jet still attached. It must be broken off and the Groove must be plowed by hand. (Exception: The Nuernberger-Rettig, q.v.) Later type casting machines generally plowed the groove automatically in the newly cast types before they were delivered.
Often the Groove is a relatively generic feature which cannot be used to identify a type, but the Grooves produced by several types of casting machines are distinctive.
There is no such thing as a standard groove. Grooves vary widely in both shape and depth, and in the end depend entirely on how the part of the caster which cut them is ground. (On the Thompson at least, the part which cuts the grooves is known, somewhat counterintuitively, as the "foot plow.")
Here is a more or less ordinary groove from a piece of 36 point ATF series no. 149 Engravers Old English Bold, cast to the "American Line" after about 1905/1906. It was probably cast on a pivotal type caster. Note the hole in the middle of the groove. This is where the typemetal had not solidified entirely before the type was delivered from the mold. It indicates a more or less substantial hole within the lower portion of the type, and is a flaw to be avoided. ATF cast good type, but not always perfect type.
At least three kinds of type casting machines produce no groove at all: The Monotype Composition Caster (American and English), the Lanston Monotype Giant Caster, and the Monotype Super Caster (English). It might be more accurate to say that they produced a Groove of zero depth. That is, they still smoothed the area where the Jet had been removed, but they did so in a way such that this area was planed flush with the rest of the foot of the type.
Here is a type from a font of 14 point Bembo cast on a Super Caster by Mouldtype in England, showing the flat bottom characteristic of type produced by that machine.
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