A common text feature is

Table of contents, index, glossary, bolded words, pictures, captions, etc

feature helps readers Visual example of feature Inset A small photo, picture, or map inside or next to a larger picture; insets magnify a part of the larger picture Helps the reader visualize something in the text in both large and small scale, in combination with the larger picture

Explanation:

b b b b b b   b b b   b b b b b