"Almost all modern English translations of the Old Testament are based on a single manuscript - the Leningrad, or St. Petersburg Codex . . . Copied in 1008 or 1009 CE, this is our earliest complete example of the traditional Hebrew Bible, or Masoretic Text.
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"Since all ancient biblical texts consisted only of Hebrew consonants without vowels, many words could be read in more than one way, leading to different readings of the same verse. Compare the letters dg in English, which could be dig, dog, or dug, depending on which vowel is used. In order to standardize the biblical text, the Masoretes added vowel signs and other components. The effect was to fix the meaning of each group of consonants (e.g., only dig, not dog or dug) . . . "
VanderKam & Flint, The Meaning of the Dead Sea Scrolls (2002), pp. 87-89.