1.Another type of RC feedback oscillator is called the twin-T because of the two T-type RC filters used in the feedback loop, as shown

2.One of the twin-T filters has a low-pass response, and the other has a high-pass response.

1. The combined parallel filters produce a band-stop response with a center frequency equal to the desired frequency of oscillation $\large f_r$

2. Oscillation cannot occur at frequencies above or below $\large f_r$ because of the negative feedback through the filters.

3. At $\large f_r$ however, there is negligible negative feedback; thus, the positive feedback through the voltage divider ( $\large R_1\: and\: R_2$ ) allows the circuit to oscillate.