Hollow looking letters in the style used for number sets in mathematics.
𝔻𝕠𝕦𝕓𝕝𝕖 𝕤𝕥𝕣𝕦𝕔𝕜
𝔻 𝕠 𝕦 𝕓 𝕝 𝕖 𝕤 𝕥 𝕣 𝕦 𝕔 𝕜
𝔻̲𝕠̲𝕦̲𝕓̲𝕝̲𝕖̲ 𝕤̲𝕥̲𝕣̲𝕦̲𝕔̲𝕜̲
Double-struck letters were drawn by mathematicians on blackboards by tracing a stroke twice, and were later standardized as the Unicode blackboard bold alphabet. They are the symbols behind the real numbers, integers and natural numbers.
The page offers plain double-struck text, a spaced version and an underlined version. Digits are also available in this style.
No. These are Unicode characters that happen to look like styled letters, so they paste into any app that accepts text, with no font installation. Because they are different characters from normal letters, search and screen readers may handle them differently.
C, H, N, P, Q, R and Z sit in a different part of Unicode than the others because they were encoded earlier, but they display as part of the same alphabet.
Yes, both are available in the double-struck set.