A Structural Grammar of the Indus Valley Script: Rebus Encoding, Positional Syntax, and the Genitive Prefix in a 4,500-Year-Old Administrative Writing System

The Indus Valley Script (c. 2600–1900 BCE) is among the last undeciphered writing systems of the ancient world. We present a structural analysis of a 282-seal corpus from Harappa (n=227) and Mohenjo-daro (n=55) revealing a systematic grammatical framework operating on rebus-encoded signs. Our principal findings are: (1) a KU+KU genitive prefix (*ku: + *ku: meaning 'of (the)') appears in 26.0% of Harappa seals and 1.8% of Mohenjo-daro seals, establishing a genitive case-marking system; (2) positional syntax analysis reveals that sign position encodes grammatical role — position-0 signs function as nominal heads while terminal positions mark objects and verbal suffixes; (3) sign reduplication marks plurality/collectivity, paralleling systems found in later Dravidian languages; (4) seventeen proto-seal clusters at Harappa — consecutive identical-seal runs such as H-261 to H-265 (5 identical seals reading KU KU KAL ALU MI AN) — demonstrate mass-production of official seals for administrative departments; (5) the TIR+TI bigram (*tir *ti: = 'star-arrow') appears 15 times exclusively at Harappa, functioning as a celestial or deity title; (6) cross-site analysis shows Mohenjo-daro uses a distinct dialectal pattern with higher formulaic diversity (55 seals = 55 unique formulas, 100% diversity) compared to Harappa's more formulaic structure (175 unique formulas from 227 seals, 77.1% diversity). These findings establish that the Indus Script is a fully functional writing system with documented grammar, supporting the Proto-Dravidian identification of its language.