Outlines can be vertical lists or horizontal columns. For example, you would use a vertical structure if your notes consisted of long paragraphs grouped under headings or if you wanted to indent items in a list. You might use a horizontal structure for a list of coworkers' telephone numbers or a list of words and their definitions.

Vertical outline structure
Indented items in a list
Horizontal outline structure
To create a vertical outline in Microsoft Office OneNote 2003, you follow a paragraph with another paragraph at the same level or indented one level. To create a horizontal outline, you press TAB to create a new column before you press ENTER.
Whether your outline is horizontal or vertical, you can use headings and subheadings to provide a structure that is useful in several respects: