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Before describing Gnome, let us introduce some terms used not only in this guide but in all Gnome documents. Most importantly, we need to clarify the use of mouse buttons and clicks.
Most Gnome documents assume that you are using a standard (for UNIX) 3-button mouse and talk about left, right, and middle mouse buttons; if a document says "click" without explicitly specifying the button, the left button is assumed. Some documents use notations "mouse button 1", "mouse button 2" and "mouse button 3" (or MB1, MB2, MB3 for short) for left, middle, and right buttons respectively.
If you are using a two-button mouse, you can emulate the middle mouse button by pressing left and right buttons simultaneoulsy; if you have a wheel mouse, the wheel can be used in place of the middle mouse button.
You can switch the roles of the buttons using Gnome Control Center. Many left-handers choose to reverse the right and left buttons. If you have done so, you need to use the right mouse button whenever document instructs you to click, and use left mouse button whenever a document talks about "right-clicking" or mouse button 3.
If you use a mouse with some unusual placement of buttons, a trackball, or some other input device, you need to find out which buttons correspond to "right", "left" and "middle"; this information can usually be found in the manual which came with your device. Usually, "left" button (MB1) is the one under your index finger.