Chalk It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are then consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacture now may utilize mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to
Blackboard bold prefers boldface to blackboard bold, and consequently did not include blackboard bold in the Computer Modern fonts he created for the TeX mathematical typesetting system. The "Chicago Manual of Style" in 1993 (14th edition) advises: "blackboard bold should be confined to the classroom" (13.14) whereas in 2003 (15th edition) it states that "open-faced (blackboard) symbols are reserved for familiar systems of numbers" (14.12). TeX, the standard typesetting system for mathematical texts, does not contain direct support for blackboard bold symbols, but the add-on AMS Fonts package (codice_1) by the American Mathematical Society provides this facility; a blackboard bold R is