A typeface used for representing bold characters, mainly used in mathematical texts, that allows these characters to be easily distinguished from non-bold characters when written.
Blackboard bold is a typeface style that is often used for certain symbols in mathematical texts, in which certain lines of the symbol are doubled. The symbols usually denote number sets. Blackboard bold symbols are also referred to as double struck, although they cannot actually be produced by double striking on a typewriter. The Chicago Manual of Style in 1993 advises: "blackboard bold should be confined to the classroom" whereas in 2003 it states that "open-faced symbols are reserved for familiar systems of numbers". In some texts these symbols are simply shown in bold type: blackboard bold in fact originated from the attempt to write bold letters on blackboards in a way that clearly differentiated them from non-bold letters, and then made its way back in 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, therefore, do not recognize blackboard bold as a separate style from bold: Jean-Pierre Serre, for example, has publicly inveighed against the use of "blackboard bold" anywhere other than on a blackboard, and 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 advises against the use of blackboard bold in print.
The numerical value of blackboard bold in Chaldean Numerology is: 7
The numerical value of blackboard bold in Pythagorean Numerology is: 3