(main test devices: MacBook Pro/ System: Mac OS X10.5.5)
After connecting external monitor correctly, under system preferred setting, call the “Monitor” setting option:
Click “Test Monitor” button to test current monitor state.
Click “Arrary” label for relevant display setting (such as mirror display or menu bar and Dock display position, and the arrange position setting of the two monitors, drag them to realize).
Click “Integrated Window”, you could set up different resolution display for the two monitors independently (cancel mirror monitor option check).
(Note: these three option settings are available before you connect the external monitor and laptop output interface correctly)

Menu bar, Dock, prompt of the related display position for cursor T1&T2 (no matter the physical position of the monitor);
T1>, array setting is vertical display mode (according with mouse operation habit of pile-up of two monitors)
T1.1+Dock, set up to display at the bottom of the screen, Dock would display—(drag) set up to display on the below monitor.
T1.2+Dock, set up to display at the left (right) part of the screen, Dock would all display—(drag) set up to display on the monitor on the display menu.
T2.1+Dock set up to display at the bottom of the screen, Dock would display—(drag) set up to display on the monitor on the menu.