What are ‘hexadecimal’ codes?

Characters are often designated by their Unicode hexadecimal value, usually in the form ‘U+nnnn’ (or ‘U+nnnnn’, depending on the length of the number). In this notation, ‘n’ represents a digit. In decimal (‘base-10’) notation, digits range from 0 to 9 (0123456789); in hexadecimal or ‘base-16’ notation, digits range from 0 to F (0123456789ABCDEF). In ‘hex’, you count 0 1 2 3 4 5 6 7 8 9 A B C D E F and then 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 – etc.
In hexadecimal numbers, it does not matter if you use capital letters A-F or lowercase a-f, or even a mix of capital and lowercase.

Hex codes in Unicode

Each character in the Unicode Standard is designated by one unique hexadecimal number. To mark a hexadecimal number as a ‘Unicode hexadecimal scalar value’, to give it its formal name, we prefix the hex number by ‘U+’. So the Unicode character ‘LATIN SMALL LETTER A’ is U+0061. Unicode hexadecimal scalar values are at least four digits long, and five-digit ones are also used regularly in Brill publications: for instance, U+1D510 designates 𝔐 (‘MATHEMATICAL FRAKTUR CAPITAL M’), a symbol often used to indicate the Masoretic text of the Hebrew bible.