What are ‘hexadecimal’ codes?

Characters are often designated by their Unicode hexadecimal value, usually in the notation ‘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 a (‘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.

How do I use these hex numbers to input characters?

You may have to insert a character, and you have no idea how to do that by just typing it using the keyboard. Supposing you saw the character ă mentioned in the ‘Brill Typeface Character List’, which shows the associated hex code 0103 next to it, how do you use that code?

MS Word on MS Windows

MS Word (Windows version only, not the macOS version) has a keyboard command to convert any Unicode hexadecimal scalar value to the associated character.