From BlenderWiki

Changing Window Frames

Maximizing a window

You can maximize a window to fill the whole screen with the View » Maximize Window menu entry (Ctrl↑). To return to normal size, use the View » Tile Window (Ctrl↓). An even quicker way to achieve this is to use ⇧ ShiftSpace to toggle between maximized and framed windows.

Splitting a window

The Split menu.

You can split a window frame (thus creating a new, smaller window) by focusing it (moving the mouse to its edge), clicking the vertical or horizontal border with MMB or RMB , and selecting Split Area (The Split menu). You can now set the new border’s position by moving your mouse to the desired position, and clicking with LMB , or you can cancel your action by pressing Esc. A click on MMB toggles between vertical and horizontal splitting. The new window will start as a clone of the window you split. You can then set it to a different window type, or to display the scene from a different point of view (in the case of the 3D window).

Joining two windows

You can join two windows into one by clicking the border between them with MMB or RMB and choosing Join Areas. Then you’ll have to click on one of the two windows – the one you click (darker with an arrow over it) will disappear, while the other will be expanded to cover the full area of both windows. If you press Esc or RMB before clicking on one of the windows, the operation will be aborted.

Obviously, you cannot join two windows if they are not on the same “level”. For example, in the (Joinable and unjoinable windows) picture, you cannot directly join one of the top windows with the bottom one, as this one shares its border with both top windows (if you try by RMB -clicking on their common border, you’ll see that the Join Areas option is not present). You must first join the two top ones, and then you’ll be able to join the resulting window with the bottom one…

Joinable and unjoinable windows.

Changing window size

You can resize windows by dragging their borders with LMB . Simply move your mouse cursor until it changes to a double-headed arrow, and then click and drag.

Application Frame

Blender allows the layout of various parts of its interface to be altered in terms of size and position of its window frames. However, when using window frame actions such as minimizing and maximizing a window frame, all actions are constrained to the current application frame dimensions (also known as the top level frame, window manager frame or frame 0), which is provided by the operating system and is placed around the Blender application as a whole. For example, if you currently have your application frame only taking up half of your screen, and want it to take up all of your screen, you would need to click on the outer application frame controls for maximizing windows, rather than using one of the possible Blender key combinations such as Ctrl↑. Using Ctrl↑ while over frame 2, for example, would only make frame 2 fill the entire space of the application frame, not the entire screen (unless the application frame was already filling the entire screen). In the screenshot below the application frame is indicated by FRAME 0 and is light blue with the title Blender in the center of it. Be aware that the application frame can be different in style, color and layout, and may not be present at all, depending on both the operating system you are running Blender in and the settings used by Blender when it is executed.

Most of the time, in this Manual, the application frame is not shown to both save space and prevent confusion as different operating systems can have different application frame layouts.