The Bf 109 G-12 was a two-seat trainer version of the Bf 109. This was a conversion of “war-weary” or rebuilt G-4 and G-6 airframes;the space needed for the second cockpit was gained by reducing the internal fuel capacity to only 240 L (60 US gal) meaning that the 300 L (80 US gal) drop tank was employed as standard equipment. This version was rarely armed with anything more than one or two cowling machine guns.The rear cockpit canopy was bowed out to give the instructor, who sat behind the student pilot in an armoured seat, a clearer view. The rear cockpit was also equipped with a basic instrument panel and all flight controls. (from Wikipedia)