Monochrome screen generations. 2001-1G-scroll wheel >> 2002-2G-touch wheel >> 2003-3G-touch button & wheel >> 2004-4G-click wheel An all 1.8inch hard disk. support.apple.com/specs/ipod/iPod_original_scroll_wheel_5... support.apple.com/specs/ipod/iPod_touch_wheel_5_10_20_GB.... support.apple.com/specs/ipod/iPod_dock_connector_10_15_20... support.apple.com/specs/ipod/iPod_Click_Wheel_20_40_GB...