this makes the button fill the whole window initially (its allocation is expandedand it is stretched to fill that allocationit also makes the button grow as the parent window is resized as shown in figure - the button in this window does expand when its parent expandsbut only along the horizontal axis figure - resizing with expand=yesfill= to make the button grow in both directionsspecify both expand=yes and fill=both in the pack callnow resizing the window makes the button grow in generalas shown in figure - in factfor more funmaximize this window to fill the entire screenyou'll get one very big tkinter button indeed figure - resizing with expand=yesfill=both adding buttons and callbacks