APL Quad Diamond

It runs one operation over each local block of an array. Put the function on the left; give the window dimensions and array on the right. A 3 3 window covers three rows by three columns. At the edges, incomplete windows use the array's fill value—the value APL supplies where cells are missing.

APL grew from Kenneth Iverson's notation for describing algorithms; it became an interactive programming language in early 1966. A 2016 stencil design exercise proposed ⌺. It chose “stencil” over “tessellate” or “tile,” because those words suggest separate regions, while this operator's windows overlap.

The proposal drew on the cut operator in SHARP APL and J, two related array programming languages; that operator inspired the design but is not another name for ⌺.

How do I type the APL stencil symbol?

On Linux, use an APL keyboard layout or a Compose sequence. Dyalog's UNIX guide notes that, as of April 2017, a Linux desktop layout might lack a default single-key chord for ⌺. It lists a Compose sequence, an XKB mapping change (XKB is Linux's keyboard-layout system), or the overstrike form in Dyalog's editor, where a symbol is entered as a sequence of characters.