Vocabulary is a principle contributor to reading comprehension, fluency, and academic achievement. Fluent readers recognize and understand many words, and read more quickly than those with smaller vocabularies. Students who have a large vocabulary understand texts better. Vocabulary words are categorized into tiers. Tier 1 words are basic words that most children know before entering school or a new grade level. Examples include clock, count, baby, call, house, and family. Tier 2 words are words that appear frequently in text, are useful and interesting, can be explained using known terms and are connected to a topic of study and/or can be associated with other words. Examples of Tier 2 words include ancient, dignified, resident, glum, and ambitious. Tier 3 words are uncommon words that are associated with specif content. Examples of Tier 3 words include subtrahend, numerator, denominator, and modulation.