Blackboard Bold
Blackboard bold is a typeface style often used for certain symbols in mathematics and physics texts, in which certain lines of the symbol usually vertical, or near-vertical lines are doubled. The symbols usually describe number sets. Blackboard bold symbols are also referred to as double struck, although attempting to produce them by double striking on a typewriter is unlikely to give satisfactory results. The symbols are nearly universal in their interpretation, unlike their normally-typeset counterparts, which are constantly reused.
In some texts, these symbols are simply shown in bold, and 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.
