The "authority" for characters is the Unicode Consrtium where you can find charts of all the characters that the standards organization knows about and has defined numeric codes for.

As a standard practice, the Unicode people give the character code numbers in hexadecimal. Most characters are defined by a "four digit" hex number, although some require "double byte" "8-digit hex."

Every hex number of course has a "decimal equivalent" and in Windows, the "Alt-Numpad" method works for "low numbers." With NumLock turned on, holding down the Alt key while you type the decimal character number on the NumPad should insert the character glyph for that decimal number. This method originated when ASCII characters were the only ones you could type, so it's sometimes called "ASCII Coding."

(Note that this method can be rather "clumsy" on laptops that don't have a discrete NumPad.)

Earlier Windows version could only use decimal numbers up to 255 with this method, but recent versions generally work with some decimal numbers up to 4-digits.

Since decimal 9999, the largest decimal character number generally usable with the Alt-Numpad method, is only 270F in hex notation (✏ which may or may not show in your browser), that leaves awholebunchofcharacters the method won't work for. For common uses, lots of those don't work anyway, but it's usually safest to use the hex numbers if you're in the least unsure of whether you'll get what you expect, and always preview and proof check anytime you do anything dangerous.

In Word, you can type the hexadecimal character number and with the cursor immediately to the right of the last digit, Alt-X should transform the character to the glyph for that hex number.

In Word, Alt-X is a "toggle" so you can put the cursor next to a character that you've pasted from someplace else, and Alt-X will show the hex number for the character. This is sometimes useful even if the character shows as an "unknown" (usually a blank square) in the source you copied it from, and may still tell you what the character was meant to be.

A character that has been inserted in Word, using the Alt-X method, generally can be copied and pasted elsewhere and nearly always will show as the correct glyph in the place where you paste it.

In all cases, the correct character glyph can only be displayed on your computer if your computer has "any font" that contains the glyph you want. Even this may not be sufficient in html, since sometimes rendering the correct glyphs requires using a "correct" encoding in your browser. There is no single encoding that "always works," mainly because lots of html-coded source materials don't stick to the standards, but generally UTF-8 is a good one to try if you're seeing a lot of "unks."

"International" keyboards exist for typing most languages (Microsoft lists and displays something like 130 or so at one of their websites) but about the only way to get one for a language is to buy it in a country where the language is fairly common.

There is a "correct" numeric code for the € symbol, for people who have a keyboard that has a key for it. For US keyboards, and others that lack the key, Microsoft redirects the Alt-0128, since decimal 128 was "undefined" and not used in ASCII codes. It's essentially a "standard shortcut."

A remaining difficulty is with languages that are written right-to-left and/or top-to-bottom. The ability to type in proper order is incorporated in some Windows systems, but requires download (and in some cases purchase) of additional software that's not included in common Western European systems. Only some Windws OS versions have been capable of adding the accessories required, although it's becoming a little easier (or so I've heard?).

For html postings, you can use the decimal character number preceded by an ampersand (&) and "pound" (#), and followed by a semicolon (;).

You can also use the (preferred) hex character number preceded by and ampersand-pound-x (&#x) and followed by a semicolon (;).

The are also a number of characters in common use that have html "familiar names," so, for example, you can use:

© to get ©

™ to get ™

and quite a few others.

John