Why did early ASCII have ← and ↑ but not ↓ or →?
Early ASCII included ← and ↑ because they were used on teletype machines for backspace and line-feed functions. ↓ and → were omitted as they had no corresponding mechanical function on those devices.
Background
- ASCII (American Standard Code for Information Interchange), finalized in 1963, is the foundational 7-bit character encoding for early computers and telecommunications.
- The original ASCII included 95 printable characters — among them the left arrow (←) and up arrow (↑), but not down (↓) or right (→) arrows.
- This asymmetry comes from ASCII's design for electromechanical teleprinters (teletype machines). On paper-based terminals, the print head could mechanically move left (backward) and the paper could advance up (scrolling), enabling overstriking for underlining. Moving right or down were the default directions — no special symbol needed.
- The left arrow served as a backspace/underline marker; the up arrow was used as an exponentiation symbol (e.g., in early programming languages like FORTRAN).
- Down and right arrows were only added later in extended character sets (e.g., IBM PC, Unicode).