The key development that led to the eventual construction of the computer was Moody's invention of a new type of flip-flop circuit, a key component of all computer systems. Moody's design used a P-N-P-N junction, consisting of a PNP and NPN transistor connected back-to-back. Most machines of the era used Eccles-Jordan flip-flops; this was originally a tube-based concept that was being used by replacing the tubes with transistors. The P-N-P-N circuit offered much higher power output, allowing it to drive a greater number of "downstream" circuits without additional amplifiers. The overall effect was to reduce, sometimes greatly, the total number of transistors needed to implement a digital circuit. Moody published his circuit in 1956.