Lexile Codes
Sometimes a Lexile measure by itself is not enough information to choose a particular book for a particular reader. This is why some books get Lexile codes—two-letter designations that appear before the Lexile measure (for example, AD580L).
The Lexile code gives you more information about a book that relates to its developmental appropriateness, reading difficulty, and common or intended usage. Word frequency and sentence length—the two text characteristics that determine a Lexile measure—do not describe all of the content of a book. Lexile codes provide some context to the numerical measure to further help you guide readers toward fruitful reading experiences.
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http://www.lexile.com/about-lexile/lexile-codes/