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ASCII

The original 7-bit character set that UTF-8 is backward compatible with.

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Also known as: ASCII table, ASCII code, American Standard Code for Information Interchange, 7-bit ASCII

ASCII (American Standard Code for Information Interchange) is the original, simple scheme for representing text as numbers. It assigns a number from 0 to 127 to each of 128 characters, which fits in 7 bits. It’s the foundation that later encodings, including UTF-8, were built on.

'A' = 65     'B' = 66     ...     'Z' = 90
'a' = 97     'b' = 98     ...     'z' = 122
'0' = 48     '1' = 49     ...     '9' = 57
' ' (space) = 32     '\n' (newline) = 10

ord("A")      # 65
chr(97)       # 'a'

What’s in the table

Range	Contents
0-31 and 127	Control characters: not printable text. Newline (10), carriage return (13), tab (9), null (0), escape (27), delete (127)
32-126	Printable characters: space, digits, uppercase and lowercase English letters, and punctuation and symbols

Notable patterns:

Digits '0'-'9' are consecutive (48-57), so ord(c) - ord("0") converts a digit character to its number.
Letters are in alphabetical order, so sorting by code point sorts alphabetically, with all uppercase before lowercase ("Z" < "a").
Upper and lowercase differ by 32, a single bit ('a' is 97, 'A' is 65), which is why some tricks can flip case with a bit operation (bitwise operations).

Limits

ASCII covers only English: no accented letters (é, ñ), no other alphabets (Cyrillic, Arabic, Chinese, Thai), no emoji, and few symbols. Different regions made extended 8-bit encodings that used the values 128-255 for their own characters, but those conflicted with each other (the same byte meant different characters in different encodings), which caused years of garbled text (“mojibake”).

Unicode solved this with one universal set of characters, and UTF-8 encodes it in a way that is backward compatible with ASCII: any valid ASCII text is also valid UTF-8, byte for byte. Characters beyond ASCII use several bytes (character encoding).

Why it still matters

Protocols and formats (HTTP headers, many file formats, source code identifiers) are largely ASCII.
Debugging: knowing that 0x41 is A, or that \r\n are 13 and 10, helps you read raw data and hex dumps.
Encoding errors: “it works for English text but breaks for names with accents” usually means an ASCII assumption somewhere.
Sorting and comparison behave differently for ASCII-order versus locale-aware order.
Security: don’t assume input is ASCII. Validate and handle Unicode properly.

When in doubt, use UTF-8 everywhere, and remember that ASCII is its simple, English-only subset.