(A) The indicated frame size corresponds with the outside format of your subject or mat. When framing, 3 to 5 mm on each side of the subject will be covered by the frame.
(B) The opening size is equal to the outside format of your subject or mat, minus 3 to 5 mm on each side.
(C) The outside measurement of your frame exceeds the "subject" size by the width of the moulding minus 3 to 5 mm (on each side).