The Sarati (sg. "sarat") of Rúmil of Valinor are a writing system devised by J. R. R. Tolkien for his Middle-Earth stories. Within the canon itself, Rúmil invented the Sarati system in Valian year 1179, to be the first writing system which existed, predating the Cirth and Feanor's Tengwar by more than seventy years.
Unlike the Tengwar, which is flexibly shifted to represent different sets of phonemes, depending on the language of the writing, the Sarati symbols each represent a single discrete phoneme which is inflexible, no matter what language uses the writing system. For every phoneme represented in the IPA, there is a single corresponding sarat, and any given language simply uses a subset of sarati containing all possible phonemes in that language.
The forms of the symbols themselves do undergo some "evolution" over time, as Tolkien depicts both "early" and "late" forms of Rúmil's work. The later Tengwar show clear evolution from the Sarati, with a few of the symbols being nearly identical between Tengwar and Sarati forms, and with the vowel diacritics being almost entirely preserved across both orthographies.
Sarati was designed to be both a scribal language on paper, and an inscribed language in stone and jewelry. It can be written in vertical columns, horizontal lines, forward, backward, and boustrophedon (in lines which alternate forward and backward). Some forms of Sarati use a guide line very similarly to the staves in Ogham script or conventional sheet music, while other forms omit the guide line completely.
According to footnotes on Tolkien's canon, Sarati was just going completely out of use (and Tengwar was beginning to go out of common use) at approximately the time of The Hobbit, partly due to the increased use of Westron writing systems as the Age of Elves drew to a close and the Age of Men began.