<P> Graphite pencils are made of a mixture of clay and graphite and their darkness varies from light grey to black: the more clay the harder the pencil . There is a wide range of grades available, mainly for artists who are interested in creating a full range of tones from light grey to black . Engineers prefer harder pencils which allow for a greater control in the shape of the lead . </P> <P> Manufacturers distinguish their pencils by grading them, but there is no common standard . Two pencils of the same grade but different manufacturers will not necessarily make a mark of identical tone nor have the same hardness . </P> <P> Most manufacturers, and almost all in Europe, designate their pencils with the letters H (commonly interpreted as "hardness") to B (commonly "blackness"), as well as F (usually taken to mean "fineness", although F pencils are no more fine or more easily sharpened than any other grade . also known as "firm" in Japan). The standard writing pencil is graded HB . This designation might have been first used in the early 20th century by Brookman, an English pencil maker . It used B for black and H for hard; a pencil's grade was described by a sequence or successive Hs or Bs such as BB and BBB for successively softer leads, and HH and HHH for successively harder ones . The Koh - i - Noor Hardtmuth pencil manufacturers claim to have first used the HB designations, with H standing for Hardtmuth, B for the company's location of Budějovice, and F for Franz Hardtmuth, who was responsible for technological improvements in pencil manufacture . </P> <P> As of 2009, a set of pencils ranging from a very hard, light - marking pencil to a very soft, black - marking pencil usually ranges from hardest to softest as follows: </P>

What do the letters hb stand for on a pencil
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