acos (arc cosine) is one of several extended math command available in [Tclx]. '''[DGP]''' Tclx?!?! What? The '''acos''' function has been supported by plain old [expr] in plain old [Tcl] since ''at least'' release 7.4. It is a procedure which provides command access to the [expr] math function, taking the same arguments as the expr function and taking expressions as arguments as well. '''[DGP]''' This sounds like more description of some ridiculously out of date thing from [TclX]. If we must still talk about such matters, let's please shove them over in a corner somewhere, and leave this page for discussing the current, standard, in plain old Tcl '''acos''' function. One good use for [acos] is to get the constant Pi: % expr acos(-1) 3.141592653589793 % tcl::mathfunc::acos -1 ;# In Tcl 8.5 3.141592653589793 ---- [Category Command] | [Category Mathematics] | [Category TclX]