The TeX primitive \pretolerance is an integer parameter that is used in TeX's line breaking algorithm as described below. The quantity <integer> is an integer from -1 to 10000.
If \pretolerance is an integer from 0 to 10000, then TeX's line breaking algorithm first attempts to break up a paragraph without hyphenation. In this attempt TeX ties to minimize the badness of each line. If TeX can break up a paragraph so that none of the lines have badness greater than \pretolerance, then TeX accepts it. If this is not possible, TeX will re-attempt to break up the paragraph with the allowance of hyphenation of words. In this second attempt, the integer parameter \tolerance is used instead of \pretolerance. If TeX is unsuccessful in the second attempt an error is reported in the log file.
If \pretolerance is -1, then TeX bypasses the first attempt at breaking a paragraph without hyphenation.