The zener effect is predominant for breakdown voltages less than 4 V, the avalanche effect is predominant for breakdown voltages greater than 6 V, and both effects are present between 4 and 6 V. Originally, people thought the zener effect was the only breakdown mechanism in diodes. For this reason, the name "zener diode" came into widespread use before the avalanche effect was discovered. All diodes optimized for operation in the breakdown region are therefore still called zener diodes.