This British Standard applies to welded and seamless, screwed and socketed steel tubes and tubulars, and to plain end steel tubes suitable for screwing to BS 21 pipe threads of nominal bores from 1/8 in (6 mm) to 6 in (150 mm) inclusive. Three thicknesses of tube are provided for as specified in Tables 1, 2 and 3 and are designated Light, Medium and Heavy.

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