The affix ac comes after the word rātri when preceded by ahan or sarva or a word denoting a portion of night, or saṃkhyāta or puṇya when the compound is a tat-puruṣa as well as when a numeral or an indeclinable precedes rātri,
The anuvritti of संख्याव्ययादेः is drawn in to this sutra by force of the word च् \and\ of the sutra. The word अहन् forms a Dvandva compound as अहश्च रात्रिश्च = अहोरात्रः \day and night\. So also सर्वरात्रः, पूर्वरात्रः 'the first part of night' : अपररात्रः 'the latter part of night'. These are formed by 2.2.1. So also संख्याता रात्रिः = संख्यातरात्रः ॥ This compound is formed by 2.1.57. So also पुण्या रात्रिः = पुण्यरात्रः । So also when compounded with a Numeral or an Indeclinable : as, द्वे रात्री समाहृते = द्विरात्रः, त्रिरात्रः, अतिक्रान्तो रात्रिम् = अतिरात्रः, नोरात्रः ॥,