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First let's define what a "Pi filter" is:

![Image alt text](https://electrical.codidact.com/uploads/CBP4y7nbtUiLqYogmGtWpgri)

The name comes from the inductor and two caps forming the shape of the Greek letter Π.

These filters are used for EMC compliance because they attenuate high frequencies.  At minimum there is a L-C filter, which attenuates by 12 dB/octave above the rolloff frequency.

The special point of a Pi filter is that they work both ways.  The power line would typically be on one side, and a device that consumes power on the other.  For EMC compliance, the high frequencies caused by the device are attenuated before getting dumped onto the power line.  In the other direction, it protect the device from nasty high frequencies that might be on the power line for whatever reason.