ASCII (American Standard Code for Information Interchange) is a character encoding standard that assigns numeric values (0–127) to letters, digits, punctuation marks, and control characters. Developed in the 1960s, it remains the foundation of modern text processing. Every time you type a letter on your keyboard, the computer translates it to an ASCII code (or its Unicode equivalent). ASCII is crucial because it provides a universal way for computers to represent and exchange text data across different systems and platforms.

ASCII uses only 7 bits, representing 128 characters (0–127), which covers basic English letters, numbers, and symbols. Unicode, on the other hand, is a much larger standard that can represent over 149,000 characters from virtually all writing systems worldwide, including emojis, mathematical symbols, and ancient scripts. The first 128 Unicode code points are identical to ASCII, making Unicode fully backward-compatible with ASCII. In modern applications, UTF-8 (a Unicode encoding) has largely replaced pure ASCII for broader language support.

On Windows, you can type any ASCII character by holding the Alt key and typing its decimal code on the numeric keypad (not the top row numbers). For example, Alt + 65 produces the letter "A" (ASCII 65). For extended characters (128–255), use Alt + the 3-digit code. On macOS, use Option key combinations or the Character Viewer. On Linux, use Ctrl + Shift + U followed by the Unicode hex code.

Control characters are the first 32 ASCII codes (0–31) plus code 127 (DEL). They are non-printable characters originally designed to control teletypewriters, printers, and communication protocols. Common examples include: NUL (0) — null terminator in C strings; TAB (9) — horizontal tab; LF (10) — line feed (new line on Unix); CR (13) — carriage return; ESC (27) — escape sequence initiator; and DEL (127) — delete. While many are rarely used today, some like TAB, LF, and CR remain essential in modern text formatting.

HTML entities are special codes used in HTML to display characters that have special meaning in the language. For example, < displays "<", > displays ">", and & displays "&". Each ASCII character can also be represented as a numeric entity like A for "A". Using HTML entities is essential when you need to display reserved characters (like angle brackets in code examples) or when your document's character encoding may not support certain symbols. They ensure your content renders correctly across all browsers.

The standard ASCII table contains exactly 95 printable characters (codes 32–126). This includes: space (32), 10 digits (48–57: 0-9), 26 uppercase letters (65–90: A-Z), 26 lowercase letters (97–122: a-z), and 32 punctuation marks and symbols spread across ranges 33–47, 58–64, 91–96, and 123–126. The remaining 33 codes (0–31 and 127) are non-printable control characters used for device control and communication protocols.