The number series of machine screws has been extended downward to include #00-90 (0.047 in = 0.060 in − 0.013 in) and #000-120 (0.034 in = 0.060 in − 2 × 0.013 in) screws; however, the main standard for screws smaller than #0 is ANSI/ASME standard B1.10 Unified Miniature Screw Threads. This defines a series of metric screws named after their major diameters in millimetres, from 0.30 UNM to 1.40 UNM. Preferred sizes are 0.3, 0.4, 0.5, 0.6, 0.8, 1.0 and 1.2 mm, with additional defined sizes halfway between. The standard thread pitch is approximately of the major diameter. The thread form is slightly modified to increase the minor diameter, and thus the strength of screws and taps. The major diameter still extends to within "H" of the theoretical sharp "V", but the total depth of the thread is reduced 4% from "H" =  cos(30°) "P" ≈ 0.541"P" to 0.52"P". This increases the amount of the theoretical sharp "V" which is cut off at the minor diameter by 10% from 0.25"H" to − ≈ 0.27456"H".