Blackboard bold

Blackboard bold is a typeface style that is often used for certain symbols in mathematical texts, in which certain lines of the symbol (usually vertical or near-vertical lines) are doubled. The symbols usually denote number sets. One way of producing Blackboard bold is to double-strike a character with a small offset on a typewriter. Thus they are also referred to as double struck.

单词	Doublestrike
释义	Doublestrike 英语百科 Blackboard bold Blackboard bold is a typeface style that is often used for certain symbols in mathematical texts, in which certain lines of the symbol (usually vertical or near-vertical lines) are doubled. The symbols usually denote number sets. One way of producing Blackboard bold is to double-strike a character with a small offset on a typewriter. Thus they are also referred to as double struck.
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Doublestrike

Blackboard bold