BS 1669:1972

Plates with round or square perforations, staggered pitch. Round holes 125 to 3.15 mm diameter, square holes 125 mm to 4 mm side of square, following the R 20 preferred number series. Five classes of round hole plate open areas 58 per cent to 23 per cent; four of square hole plates open areas 64 per cent to 31 per cent.