Transitional Serifs evolved from a conscious departure from Humanist Serifs. They are much less influenced by handwritten letterforms and are more mechanical, tending toward stronger vertical axes and horizontal stress.
Additionally, typefaces in this classification have greater
variation in stroke widths.
Transitional Serif typefaces gained a lot of momentum at the beginning of the 18th century. However, the first typeface considered to be a Transitional Serif was the Romain du Roi, or the King's Roman, created by Grandjean in the 1690s.
During the 18th century, the era of Enlightenment was marked by resistance, so it stands to reason that typographers wanted to design typefaces far removed from humanist calligraphic traditional.
Typeface examples: