一共 3 行，前两行，从字形上，就是普通的 math italic 字形，而第 3 个公式是黑板粗（blackboard bold）字体。当然，图一的黑板粗比较特殊，算是有点加粗的感觉，在一些数学字体宏包中，. The numbers $1$ to $2025$ are written on a blackboard and the following process is repeated for as long as there are enough numbers left: 一、黑板粗体 (blackboard bold) \mathbb实际上指的是 黑板粗体 (blackboard bold)，在原先的字母基础上多了一些特定的线条，所以有时候也被称为 双线体 (double struck)。
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Young authors (and some older ones) seem to prefer blackboard bold to plain bold for the standard number systems (and/or they pick it up by osmosis on web sites such as this. Does anyone know a good resource (preferably pictures) that illustrates a conventional way to write the special sets symbols, i.e. 该字体称为 blackboard bold（黑板粗体），可使用 \mathbb{} 命令打出。 p.s.
$\\bbb p$ i am familiar.
In the first post of the thread "cardinal number subtraction", cardinal number subtraction there is a symbol for some kind of set which looks like this:
