About nested framesets
A frameset inside another frameset is called a nested frameset. A single frameset file can contain multiple nested framesets. Most web pages that use frames are actually using nested frames, and most of the predefined framesets in Dreamweaver also use nesting. Any set of frames in which there are different numbers of frames in different rows or columns requires a nested frameset.

Example
For example, the most common frame layout has one frame in the top row (where the company’s logo appears) and two frames in the bottom row (a navigation frame and a content frame). This layout requires a nested frameset: a two-row frameset, with a two-column frameset nested in the second row.