This is documentation for Mathematica 7, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.2)
Low-Level Notebook Structure
Like everything else in Mathematica, notebooks are ultimately symbolic expressions. When you edit notebooks—or apply high-level programmatic functions—Mathematica automatically updates these expressions. But if you look at the lowest level—say by opening a notebook file as text—you will see the underlying expressions, in which formatting constructs are represented as a hierarchy of low-level symbolic "boxes".
Ctrl+Shift+E toggle between formatted display and underlying symbolic expression
Notebook low-level symbolic representation for a notebook
Cell low-level representation for a cell in a notebook
RowBox low-level representation for a row of arbitrary elements
GridBox low-level representation of an arbitrary 2D layout
StyleBox low-level wrapper specifying styles and style options to apply