Blackboard bold is a style of writing bold symbols on a blackboard by doubling certain strokes, commonly used in mathematical lectures, and the derived style of typeface used in printed mathematical texts. The style is most commonly used to represent the number sets , (integers), , , and .
Blackboard bold used on a blackboard
Typewritten lecture notes by Gunning (1966), showing "blackboard bold" style R and C achieved by double-striking each letter with significant offset.
A page from Loomis & Sternberg (1968), showing an early example of "blackboard bold" style R and C in a printed book.
A blackboard or a chalkboard is a reusable writing surface on which text or drawings are made with sticks of calcium sulphate or calcium carbonate, known, when used for this purpose, as chalk. Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.
NASA scientist Homer E. Newell Jr. explaining principles of altitude, pressure, and temperature, c. 1973
A chalkboard with an eraser in the chalk tray
Stick-slip effect with a chalk on a blackboard
Middle school chemistry on a blackboard in Beijing, China, 2011