The Indus script was used from 2600-1900 BC in the Harappan civilization and was written from right to left with 400-600 principal signs. Scholars believe it was a logo-syllabic script, but it remains undeciphered due to a lack of bilingual texts for comparison and the short lengths of inscriptions, with the longest being only 27 signs; there have been many attempted decipherments but none have proven correct.