This document discusses tolerances in engineering drawings. It defines tolerances as acceptable amounts of dimensional variation that allow objects to still function correctly. There are three basic tolerance types - limit dimensions, unilateral tolerances, and bilateral tolerances. Limit dimensions specify the largest and smallest allowed dimensions. Unilateral tolerances allow variation in one direction from a target dimension, while bilateral tolerances allow variation in both directions. The document also discusses general tolerances, total tolerance, measuring device accuracy in relation to tolerances, and three types of fits between mating parts.