Diode–Transistor Logic (DTL) is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors; it is the direct ancestor of transistor–transistor logic. It is called diode–transistor logic because the logic gating function (e.g., AND) is performed by a diode network and the amplifying function is performed by a transistor (contrast this with RTL and TTL).
With the simplified circuit shown in the picture the voltage at the base will be near 0.7 volts even when one input is held at ground level, which results in unstable or invalid operation. Two diodes in series with R3 are commonly used to lower the base voltage and prevent any base current when one or more inputs are at low logic level. The IBM 1401 used DTL circuits almost identical to this simplified circuit, but solved the base bias level problem mentioned above by alternating NPN and PNP based gates operating on different power supply voltages instead of adding extra diodes.
