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# Understanding Layout

Panels in GWT are much like their counterparts in other user interface libraries. The main difference lies in the fact that they use HTML elements such as DIV and TABLE to layout their child widgets.

## RootPanel

The first panel you're likely to encounter is the RootPanel. This panel is always at the top of the containment hierarchy. The default RootPanel wraps the HTML document's body, and is obtained by calling RootPanel.get(). If you need to get a root panel wrapping another element in the HTML document, you can do so using RootPanel.get(String).

## CellPanel

CellPanel is the abstract base class for DockPanel, HorizontalPanel, and VerticalPanel. What these panels all have in common is that they position their child widgets within logical "cells". Thus, a child widget can be aligned within the cell that contains it, using setCellHorizontalAlignment() and setCellVerticalAlignment(). CellPanels also allow you to set the size of the cells themselves (relative to the panel as a whole) using CellPanel.setCellWidth and CellPanel.setCellHeight.

## Other Panels

Other panels include DeckPanel, TabPanel, FlowPanel, HTMLPanel, and StackPanel.

## Sizes and Measures

It is possible to set the size of a widget explicitly using setWidth(), setHeight(), and setSize(). The arguments to these methods are strings, rather than integers, because they accept any valid CSS measurements, such as pixels (128px), centimeters (3cm), and percentage (100%).