What is an Alt code?

An Alt code is a number typed on the numeric keypad while the left Alt key is

held down. Releasing Alt inserts the character that number maps to. The mapping comes

from the IBM PC code pages, not from Unicode: three-digit codes use OEM code page 437

and four-digit codes with a leading zero use the Windows-1252 code page. Because

those two tables only hold 256 characters each, Alt codes stop at 255 while Unicode

continues into the hundreds of thousands.

Two code pages, two tables

Range	Name	Table	Size
Alt+0128 to Alt+0255	ANSI codes	Windows-1252	256 slots
`Alt` + decimal, up to 65535	Decimal code point	BMP subset of Unicode	65,536 slots
`Alt` + `+` then hex	Hex code point, registry opt-in	Full Unicode	1,114,112 slots

The first two rows are the classic Alt codes. The bottom two rows are not code pages

at all: they pass a raw Unicode code point through the same key input path.

Code page 437 and Windows-1252 both came from ASCII, so the first 128 slots agree.

Above 127 they were filled in independently. A degree sign is at position 248 in code

page 437 and at position 176 in Windows-1252, which is why both Alt+248 and

Alt+0176 produce °. A heart suit exists only in code page 437, so it has one code,

Alt+3, and no four-digit form.

For characters that exist in neither code page there is no legacy Alt code at all.

A check mark is U+2713, which is well past both tables, so there is no three-digit

or four-digit Alt code for it. What does work is Alt + 10003, the decimal code

point, on a keyboard that has a numeric keypad.

Quote whichever one the destination understands, and quote both when you are writing

for an audience that might be on either. A support article that says "type Alt+0176"

is useful to a Windows desktop user and useless to a Mac user. The same article saying

"the degree sign is U+00B0; on Windows use Alt+0176, on macOS use ⌥⇧8, in HTML

use °" is useful to all three. Every symbol page on this site quotes all of them

so you never have to translate.

The characters in the 1 to 31 range have no printable character in Unicode. Code page

437 assigned a picture to each of those slots and the OEM console font drew it, which

is where the well-known Alt+1 smiley and Alt+3 heart come from. Those keystrokes

still produce a picture in a console window using a raster font. In a modern

application drawing with a TrueType font, the same input can produce nothing visible,

because the code point being inserted is a control character rather than a picture.

No. The right Alt key is usually mapped to AltGr for composing accented characters.

Only the left Alt key starts a numeric Alt sequence.

Below 128 they are. ASCII defines the first 128 slots of every code page, so Alt+65

gives A for the same reason ASCII 65 is A. Above 127 they diverge, which is the whole

reason the two Alt code tables disagree.

Yes. The numeric keypad input path is still part of the Windows text input stack. What

has changed is the font: a modern font can only draw the picture for a control code if

the application asks for it to be drawn.