This document discusses parallel structures used when comparing two things. It notes that comparisons must use parallel structures like "-er than", "more than", "less than", "as...as", "the same...as", or "similar to". Examples are provided of sentences correctly using parallel structures to compare two things, such as comparing a school to another school or comparing the size of a car to a house. The document emphasizes that when finding things being compared in a sentence, their structures must be parallel.