The operator was originally introduced as reduced sum by Sundaram Seshu in 1956, studied as operator
∗ by Kent E. Erickson in 1959, and popularized by Richard James Duffin and William Niles Anderson, Jr. as parallel addition or parallel sum operator
: in mathematics and network theory since 1966. While some authors continue to use this symbol up to the present, for example, Sujit Kumar Mitra used
∙ as a symbol in 1970. In applied electronics, a
∥ sign became more common as the operator's symbol around 1974.[nb 1][nb 2] This was often written as doubled vertical line (||) available in most character sets, but now can be represented using Unicode character U+2225 ( ∥ ) for "parallel to". In LaTeX and related markup languages, the macros
\parallel are often used (and rarely
\smallparallel is used) to denote the operator's symbol.
The same principle can be applied to various problems in other disciplines.
Suggested already by Kent E. Erickson as a subroutine in digital computers in 1959, the parallel operator is implemented as a keyboard operator on the Reverse Polish Notation (RPN) scientific calculators WP 34S since 2008 as well as on the WP 34C and WP 43S since 2015, allowing to solve even cascaded problems with few keystrokes like 270↵ Enter180∥120∥.