The place command lets you specify the width and height of a window, and the X and Y locations of the window's anchor point. The size and location can be specified in absolute or relative terms. Relative specifications are more powerful. Example 25-1 uses place to center a window in its parent. You can use this command to position dialogs that you do not want to be detached top-level windows:
place $w -in $parent -relx 0.5 -rely 0.5 -anchor center
The -relx and -rely specify the relative X and Y positions of the anchor point of the widget $w in $parent. A relative X (or Y) value of zero corresponds to the left (or top) edge of $parent. A value of one corresponds to the right (or bottom) ...