Over the years, Apple has made changes to the methods for resizing windows.
Information in this article applies to OS X Lion (10.7) and later.
As the cursor nears the edge of the window, it changes to adouble-ended arrow.

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When you see the double-ended arrow, click and drag to resize the window.
Resizing also works on the corners of a window.
Move your cursor to any corner to produce the double-ended arrow.

Then click and drag.
Although most windows can be resized, some windows are not resizable.
To return the window to its original size and placement, press thegreen buttonagain.

To return it to its original size and position, tap the thumbnail of the screen on the Dock.
Go toSystem Preferences>Dockand check the box next toDouble-click a window’s title bar to.
Select eitherzoomorminimize, depending on how you want the setting to work.
Repeat the process with another app until two apps share the screen in equal spaces side by side.
The process for entering Split View mode varies depending on the Mac operating system.
The options for resizing windows in Split View limited.
As you make one window narrower, the other window fills the space and become wider.