The Protestant Old Testament consists of the same books as the Tanakh, but the order of the books is different. The Catholic and Orthodox Old Testaments contain six books not included in the Tanakh; see apocrypha and deuterocanonical books.
The exact number of the Old Testament books depends on whether certain disputed books are included, of which all Christian groups agree on 39 books. (The Jewish tradition counts them as 24 books. Samuel, Kings, and Chronicles as one each. The 12 minor prophets as one book rather than 12.) The Roman Catholic and Eastern Orthodox include an additional 15 books, called the Deuterocanonical books, which Protestants exclude as apocryphal.
The Old Testament text used by the earliest Greek-speaking, Christians was the Septuagint, a Greek translation that was widely held by Jews in the first century to be authoritative and which included the Deuterocanon.
** Some editions include deuterocanonical passages in this book that are omitted from other editions, or included separately as Apocrypha.
The naming of the Old Testament
Christians call this group of books the Old Testament, because of a belief (taught in the Epistle to the Hebrews) that there is a new covenant or testament between God and mankind, after the coming of Jesus.
Jews themselves do not accept the New Testament or the characterization of the Tanakh as the Old Testament (although many Jews accept Jesus as a historical figure and even as a student of a Tannaitic Sage).
The relationship between the Old Testament and the New Testament is controversial among Christians. There is some debate among Protestant scholars over the issue of whether the New Testament applies to Jewish people, but there is very little debate over its applicability to Gentiles. Similarly, the degree to which the Old Testament and its laws applies to Christians. Very few Christians, for example, follow the dietary laws within the Old Testament, whereas almost all Christians believe that the Ten Commandments are applicable. The question of which Old Testament laws are applicable affects debates on homosexuality and the ordination of women in the priesthood.
Some historical groups such as Gnostics have gone so far as to assert that the God of the Old Testament is a different being that the God of the New Testament. Most Christian groups believe that this view is heresy.
Thus, some scholars prefer Hebrew Bible as a term that covers the commonality of the Tanakh and the Old Testament while avoiding sectarian bias.
The New Testament text however does contain many references to the Old Testament, especially in relation to the fulfillment of prophecies concerning the promised messiah, whom Christians believe to be Jesus. In Christian theological views this expectation, present fulfillment and eschatological fulfillment of the divine, eternal kingdom under the headship of Christ are the thread running through both Testaments.