The '''S''' combinator is a sort-of generalised function application from combinator theory, the other most basic one being [K]. proc S {f g x} { $f $x [$g $x] } See [Hot curry] or [Combinator engine] for much more. [[Can someone provide some context here on this page as to how this would be used ''in the real world''?]] [RS]: It certainly is less evidently usable than [K], but rather of theoretical interest (like e.g. particle physics) - one of the smallest building blocks of functions. You can for example compose identity out of S and two instances of K: I = S K K but in the "real Tcl world" one would of course code identity (which is useful at times) as proc I x {set x} ---- [Category Concept] | [Arts and crafts of Tcl-Tk programming]