The main objective of this paper is to propose the first tentative periodisation of the Old Tibetan (OT) language based on a group of related sound changes. As it occurs, at the time of the script invention in the 630s, Early Old Tibetan (EOT) must have had four onset clusters /s/+liquid: zr-, sr-, zl-, and sl-. However, in Old Tibetan as well as in Classical Tibetan (CT) we only find sr-, zl-, and sl-, whereas neither of them is attested in modern spoken varieties of Tibetan. In order to find out what has happened to the EOT zr-, I have traced reflexes of CT sr-, zl-, and sl- in modern dialects. Since changes that have occurred with respect to zl- and sl- parallel each other, I postulate that the same analogy can be applied to sr- to determine in what direction the EOT onset zr- might have evolved. Having reconstructed the development of the onsets in the most conservative dialects of Western Archaic Tibetan (WAT) and Amdo Tibetan (AT), I juxtapose these findings with historical facts that can help us to explain modern distribution of Tibetan dialects. Historical events recorded in OT documents combined with our knowledge of other early sound changes in Old Tibetan constitute a time frame for dating the reconstructed changes and thereby allow us to establish the first tentative linguistic periodisation of Old Tibetan.