When you select the Rectangle Tool from the toolbar, under the Options section you’ll see an odd little button that looks like a curved right angle marked by little crosshatches. This is the Rectangle Settings button, and the only option available on the Rectangle Settings is the Corner Radius.
The Corner Radius defines just how much the corners of the rectangle will curve, by defining the radius of the arc of that curve. When it’s set to 0, the curve has no radius, so it bends at a sharp right angle—the default setting.
