Type ESC [ [ ESC and ESC ] ] ESC, and you get "thick" square bracket characters instead, which looks less silly. I think it's clear enough where the [[...]] ca

Type ESC [ [ ESC and ESC ] ] ESC, and you get "thick" square bracket characters instead, which looks less silly. I think it's clear enough where the [[...]] came from: Wolfram wanted "a b" to mean "a times b", which means "a (b + c)" has to mean "a times (b plus c)", which means you need a different notation for function application; they chose square brackets, which means they aren't available for subscripting (because everything, functions included, is an expression, and expressions are subscriptable); so they needed something else. And {...} was already taken for list construction. When what you're doing in Mathematica looks more like mathematics than programming, you don't need a lot of subscripts. Presumably that's why subscripting is the thing that got the ugly notation with extra characters. (The thing I find most unpleasant about programming in Mathematica is the way it tries to use pattern-matching as the single fundamental tool. This gives Mathematica programming, for me, something of the same feel as getting a macro-expansion based system like bash or TeX to do tricky things. But I'm not all that expert with Mathematica, and it's possible that more experienced users happily internalize the details and don't get confused as I do.)