DC characteristic #1: Input voltages (VIH and VIL)
The voltage at which an input signal is determined to be High or Low is called threshold voltage.
DC characteristics of the 74VHC04FT
Example: Input voltages (VIH and VIL) vs supply voltage (VCC) of the 74VHC04
The VIH and VIL of CMOS logic ICs are designed to be 1/2 VCC.