DAYLIGHT OPENING:      The area of glass visible in a glazed sash.  For Standard Storm Sash the GLASS SIZE is 5/16" wider and 5/16" higher than the daylight opening. 

GLASS OPENING:      The glass opening is measured glass rabbet to glass rabbet.  For Standard Storm Sash the GLASS OPENING is 3/8" wider and 3/8" higher than the DAYLIGHT OPENING

GLASS SIZE:  The actual Glass Size is 1/16" less in both Width and Height than the Glass Opening to provide clearance for the glass.  For STANDARD SIZE 2-light Storm Sash the GLASS SIZE is even inch width x even inch height; i.e. 24 x 28.