Matsya Purana — Catalogue of the Eighteen Puranas
अष्टादशभ्यस्तु पृथक् पुराणं यत्प्रदिश्यते विजानीध्वं द्विजश्रेष्ठास् तदेतेभ्यो विनिर्गतम् //
aṣṭādaśabhyastu pṛthak purāṇaṃ yatpradiśyate vijānīdhvaṃ dvijaśreṣṭhās tadetebhyo vinirgatam //
O best of the twice-born, know that the Purāṇa taught as distinct from the eighteen has nonetheless issued forth from them, from that very Purāṇic tradition.
This verse is not about Pralaya; it clarifies Purāṇic textual lineage—how a “separate” Purāṇa is still rooted in the established eighteen-Purāṇa tradition.
Indirectly, it grounds dharma-instructions in recognized scriptural transmission: royal and household duties taught elsewhere in the Purāṇa are presented as part of an authoritative Purāṇic corpus rather than an isolated teaching.
No specific Vāstu or ritual rule is stated; the takeaway is methodological—Vāstu, iconography, and rites taught later are framed as emanating from the broader Purāṇic canon, strengthening their traditional legitimacy.