Blackboard ‘chalk’ are produced especially for use with blackboards in white and also in various colours. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk rock or limestone, while colored or pastel chalks are made from calcium sulfate in its dihydrate form, CaSO·2HO, derived from gypsum. Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO (calcite). As compared to whiteboards, blackboards still have a variety of advantages: On the other hand, chalk produces dust, the amount depending on the quality of chalk used. Some people find this uncomfortable or may be allergic to it, and according
Blackboard bold from the attempt to write bold letters on blackboards in a way that clearly differentiated them from non-bold letters i.e. by using the edge rather than point of the chalk. It then made its way back into print form as a separate style from ordinary bold, possibly starting with the original 1965 edition of Gunning and Rossi's textbook on complex analysis. Some mathematicians do not recognize blackboard bold as a separate style from bold. Jean-Pierre Serre uses double-struck letters when writing bold on the blackboard, whereas his published works consistently use ordinary bold for the same symbols. Donald Knuth also