Underpoint

An object appears to present a difficulty to a flame that cannot surround it, or approach the so doing. This a mere upper-surface ignited affords little prospect of doing, an undersurface much more, an under edge still more, and an underpoint (never mind Euclid's definitions) again still more, being capable of being nearly surrounded by the smallest flame.

Source: wiktionary

Every workman who filed the teeth had the model of a perfect tooth with him and was told to file the tooth to the shape of the underpoint of a man's little finger.

Source: wiktionary

Then he straightens up his right arm, which he swings quickly as a pivot to speed a left hand punch which travels from below the outside of his slightly bended left knee to the underpoint of the opponent's chin.

Source: wiktionary

In larger dimensions, recurrent formulas, articulation, and phrase structure become important, and it may be possible to abstract various general curve types by studying the location of peaks and underpoints.

Source: wiktionary