A sub menu item, like a main menu item, consists of a table cell and a link in the cell; you may also have the ‘>’ character. The background color of this cell is changed when the mouse pointer comes over it.
A sub menu is a table of one column. Each row has one cell, and that cell has one link (one A element).
Ordinarily, a drop down menu is also a table of one column. Remember, drop down menus are also sub menus. I use the phrase here to make the distinction between what you get from a main menu item and what you get from the drop down menu itself.
When a drop down menu would produce a sub menu, the drop down menu is made up of a table of tables. In this case, you have one table with only one row (with its cells). The content of each of these cells is a table. Each of these tables is a sub menu.
In the case where a sub menu appears lower within the sub menu area, the table has another table; we shall see the details and reason for this later.
As I have been saying, there is a main table for the sub menus. This main table has only one row. In our example, there are five cells in the table (corresponding to the five main menu items above it). The content of each of these cells is the table for a drop down menu.