It is not clear that the AV translators favoured the LXX or the Vulgate over the Hebrew text.
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Modern scholarship holds that the LXX was written during the 3rd through 1st centuries BCE.
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Only two of these translations are found in the extant LXX.
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Further, the LXX symbol is also the Roman numeral for 70.
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Thus, LXX is often used as an abbreviation for septuagint.
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These issues notwithstanding, the text of the LXX is generally close to that of the Masoretes and vulgate.
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This length distinction is also found in the LXX.
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Thus this combined text became the first major Christian recension of the LXX, often called the Hexaplar recension.
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This instance illustrates the complexity of assessing differences between the LXX and the Masoretic Text as well as the Vulgate.
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More examples
Seventy: being ten more than sixty
Seventy: the cardinal number that is the product of ten and seven
The Septuagint , or simply "LXX", referred to in critical works by the abbreviation '''''', is the Koine Greek version of the Hebrew Bible, translated in stages between the 3rd and 2nd Centuries BC in Alexandria. It was begun by the third century BC and completed before 132 BC.
In Roman numerals, the number seventy
Roman symbol for the number 70. An abbreviation used to refer to the Septuagint translation of the Hebrew Scriptures (Old Testament) into Koine Greek. The translation was allegedly made by 70 or 72 individuals. ...
The Septuagint, so-called because of the tradition that it was translated by seventy scholars working independently, who all came up with exactly the same Greek wording for the original Hebrew text. "LXX" is the Roman numeral for seventy.
Abbreviation for the document known as the Septuagint.