The output voltage is 63.3% of its maximum at the critical frequencies.

A band-stop filter passes frequencies between its lower and upper critical frequencies and rejects all others.

A band-pass filter rejects all frequencies within a band between a lower and an upper critical frequency and passes all others.

In an RL low-pass filter, the output voltage is taken across the inductor and the output lags the input.

In an RC low-pass filter, the output voltage is taken across the resistor and the output lags the input voltage.