You need the -geometry option, which is described here:
http://www.wellho.net/forum/The-Tcl-programming-language/Tk-Geometry-strings.html

To be able to place a dialog on top of another window, you'll need to use the winfo command to get the information. In Perl, you'll want to do something like:
$mw->winfo("geometry")

See all possible winfo strings here: http://www.wellho.net/forum/The-Tcl-programming-language/Tk-Geometry-strings.html

In reply to Re: Tk - Place DialogBox in middle of top window by Sinistral
in thread Tk - Place DialogBox in middle of top window by Dirk80

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