Double strike letter.

I have always thought that double strike letters like

,

,

were some smart invention to give sets we find so incredibly important a clever distinguished notion. It turns out the evolution of this notation was quite a coincidence. These letters originated from the blackboard – formerly the sets of natural, real, rational numbers and so on were denoted by bold letters (which you can actually still find in old textbooks). However, writing bold letters on the blackboard – well that’s beyond our artistic skills (or just very inconvenient) and therefore people started to write double strike letters on the blackboard to represent bold letters. And since mathematicians are always short of symbols, people adapted these letters to printed texts too. Now this explains the command \mathbb in

– math blackboard bold. Some people think it is not ok to use this notation, there is even an entire

Wikipedia article about this subject.