The Arc tool, which creates circular arcs of any angle, has six modes. Create an arc by radius, three points, tangent, two points and center, two points and radius, or two end points and another point on the arc.
Either drag the mouse to draw the arc or use the Data bar to enter an angle. Degrees start on the positive X axis (the “East” direction), and increase going counter-clockwise. (“East” is 0 degrees; “North” is 90 degrees; and “West” is 180 degrees.) To draw a quarter-circle wall beginning at 0 degrees and ending at the 6 o’clock position, enter -90 (minus 90) degrees. Enter 270 to draw an arc three-quarters of a circle around.
To edit an arc, click the middle handle with the 2D Selection tool and drag to change the arc radius. Press the Option key (Macintosh) or Alt key (Windows) to change the arc’s size.