परिप्रत्युपापा वर्ज्यमानाहोरात्रावयवेषु

The particles 1. परि 2. प्रति 3. उप and 4. अप preserve their accent before that word which specifies an exclusion or a portion of day and night (in an अव्ययीभाव समास also).
Thus प꣡रित्रिगर्त्ते वृष्टो देवः "It rained all round (but not in) Trigarta". (See 2.1.11 and 2.1.12. So also प꣡रिसौवीरम् प꣡रिसार्वसनि, प꣡रिपूर्वरात्रम् ॥ So also प्र꣡तिपूर्वाह्णम्, प्र꣡त्यपराह्णम्, प्र꣡त्यपररात्रम्, उ꣡पपूर्वाह्णम्, उ꣡पापराह्णम्, उ꣡पपूर्वरात्रम्, उ꣡पापररात्रम् ॥ अ꣡पत्रिगर्ते वृष्टो देवः, अ꣡पसौवीरम्, अ꣡पसार्वसेनी 2.1.11 and 2.1.12.

By Phit Sutras¬ IV. 12, and 13 all Particles (Nipata) have acute on the first syllable. So also upasargas with the exception of अभि ॥ Therefore परि &c, have acute on the first. In a Tatpurusha and Bahuvrihi compounds, these words 'pari', 'prati' &c, as first members would have retained their accent by the rules already gone before; the present sutra, therefore, extends the principle of the preservation of the accent to Avyayibhava compounds also. The prepositions अप and परि alone denote the limit exclusive or वर्जन, and it is therefore with these two prepositions only that the second member can denote the thing excluded, and not with प्रति and उप ॥ With these prepositions अप and परि, the second term if denoting a member of day or night, are also taken even as excluded, therefore no separate illustrations of those are given.

Why do we say 'before a word which is excluded, or is a part of a day and night'? Observe प्रत्यग्नि शलभाः पतन्ति ॥ In परिवनम् "all round the forest, but not in it', the accent falls on the last by 6.2.178, which debars this general rule, as well as all special rules which might affect वन in a compound.