From Tolkien Gateway
The Andaith or "long mark" was the long-vowel diacritical mark used in Tengwar of the Mode of Beleriand. In this mode of 'full writing', vowels were represented by separate letters rather than as tehtar so a different means was needed to indicate long vowels. The andaith appeared as an 'acute accent'.
OS andatektha and also perhaps and + taith.
- ↑ J.R.R. Tolkien, The Lord of the Rings, Appendix E, "Writing", "The Fëanorian Letters"