The rough opening is a measurement term used in construction. It is the measurement from stud to stud in a new construction project. When Rough Opening Measurements are given to a manufacturer, they will make the necessary deductions in size to allow the window to fit in its opening. Remember that in measuring a window, the first number is always the width and the second is height.
For example, your contractor measures a 36″ x 36″ opening. He or She may also tell you a 3/0 x 3/0 opening which translates to (3 x 12 inches) = 36 inches by (3 x 12 inches) = 36. The manufacturer will make the window a NET SIZE of 35-1/2″ x 35-1/2″
Another example is a 36″ x 42″ opening. This could also be a 3/0 by 3/6 opening which translates to ( 3 x 12 inches) = 36 by (3 x 12 inches, plus 6 inches) = 42. The manufacturer will make the window a net size of 35-1/2 x 41-1/2″.
Deductions are usually a 1/4″ taken off each side.