अप-परी वर्जने

The force of अप and परि is in this case that of 'with the exception of.' As अपत्रिगर्त्तेभ्यो वृष्टो देवः it rained outside of or with the exception of Trigarta. So also परि त्रिगर्तेभ्यो वृष्टोदेवः. In this sense they govern the noun excluded in the ablative case 2.3.10. When not having these senses, they are not karma-pravachaniya. As ओदनं परिषिञ्चति. He waters the rice. Here it is an upasarga, and hence changes the स into ष.,