<P> The 1968 Vienna Convention on Road Signs and Signals allows for two types of stop sign, as well as three acceptable variants . Sign B2a is a red octagon with the inscription "STOP" in white . Sign B2b is a red circle with a red inverted triangle with either a white or yellow background, and the inscription "STOP" in either black or dark blue . The Convention allows for the word "STOP" to be in either English or the national language of the particular country . The finalized version by the United Nations Economic and Social Council's Conference on Road Traffic in 1968 (and in force in 1978) proposed the standard stop sign diameters of 600, 900 or 1200 mm . </P> <P> The United Kingdom and New Zealand stop signs are 750, 900 or 1200 mm, according to sign location and traffic speeds . </P> <P> In the United States, stop signs have a size of 750 mm across opposite flats of the red octagon, with a 20 mm white border . The white uppercase letters in small caps forming the stop legend are 250 mm tall . Larger signs of 900 mm (35 in) with 300 mm (12 in) legend and 25 mm (7⁄8 in) border are used on multilane expressways . Regulatory provisions exist for extra-large 1,200 mm (47 in) signs with 400 mm (16 in) legend and 30 mm (1 ⁄ in) border for use where sign visibility or reaction distance are limited, and the smallest permissible stop sign size for general usage is 600 mm (24 in) with a 200 mm (7.9 in) legend and 15 mm (5⁄8 in) border . The metric units specified in the US regulatory manuals are rounded approximations of US customary units, not exact conversions . The field, legend, and border are all retroreflective . In the Vienna Convention on Road Signs and Signals of the UN, the instruction on the sign to stop is specified to be either in English as stop or written in the local language . Some countries use both . The sign's distinctive design was developed and first used in the U.S., and later adopted by other countries and by the U.N. Despite this, the US is not a signatory to this UN Convention . </P> <Ul> <Li> <P> B2a </P> </Li> <Li> <P> Acceptable variant of B2a </P> </Li> <Li> <P> B2b </P> </Li> <Li> <P> Acceptable variant of B2b </P> </Li> <Li> <P> Acceptable variant of B2b </P> </Li> <Li> <P> Acceptable variant of B2b </P> </Li> </Ul>

What shape is a standard stop sign according to the highway code