On graphical displays, you can use window dividers in order to separate windows visually. Window dividers are bars that can be dragged with the mouse, thus allowing you to easily resize adjacent windows.
To toggle the display of window dividers, use the command M-x window-divider-mode.
To customize where dividers should appear, use the option
window-divider-default-places. Its value should be either
bottom-only (to show dividers only on the bottom of windows),
right-only (to show dividers only on the right of windows), or
t (to show them on the bottom and on the right).
To adjust the width of window dividers displayed by this mode
customize the options
For more details about window dividers see Window Dividers.