For example 3.3V TTL logic shares the 0.8V and 2V thresholds for logic 0 and logic 1 transitions with 5V TTL logic, so a 3.3V TTL output can drive a 5V TTL input without any extra hardware required.

Thus a 5V TTL input will detect logic levels from a 3.3V output while the CMOS version will not, and many an engineer including us has been caught by using the wrong chip to interface between ...