See the splitter documentation
This is the left side of the vertical splitter. Typically you might use this side for a navigational display of some data that is related to the right side.
Using CSS styles you can control the look of the splitter, such as its color, width, and appearance when selected. Or, you can specify most options when you call the plugin to create the splitter.
This is the right side of the vertical splitter. Usually this side will be wider than the left side, but that's up to you. The left pane of this demo was set to be 150 pixels wide in the stylesheet, which is why it's smaller than the right side. If you don't specify a width for either panes when the splitter is created, the two panes will be equal width.
To move the splitbar using the keyboard, press Alt-Shift-I (Mac users, Cmd-Shift-I), then press the arrow keys. To unfocus the splitbar, press Tab.