Though they're not interchangeable, of course, due to the different interpolation behaviour.

What I found amusing, though, was when I learned that except for the interpolation, they are identical - in particular, you can use quotation marks for "blocks" (arguments to things like if or proc or the like), if there are no interpolatables in the block (or you escape them yourself). Even multi-line ones.

That was simply something I wasn't used to, coming from C-like languages: blocks had braces and strings had quotes, and never the twain shall meet - but in Tcl, essentially everything's a string. (Or everything's a list of strings? At any rate, "blocks" aren't special.)