In the first century CE the Jewish scriptures were not bound in a book, but were written on scrolls that contained one or more texts. Jewish scholars referred to these scrolls as the Law (Torah), the Prophets (Nebi'im), and the Writings (Kethubim). Today Jews often refer to their scripture as the Tanakh, an acronym using the first three Hebrew letters for these groups of scripture.
In urban synagogues during the time of Jesus there would have been readings on the sabbath from scrolls in Hebrew. In rural Sabbath gatherings the readings would have been from Targums, commentaries on the scriptures in Aramaic, the spoken language of the people of Galilee.
Paul and the other authors of the New Testament read these Jewish scriptures in Greek. In the first churches within Roman cities the only written scripture was the Septuagint, the Greek translation of the Hebrew scriptures that was read throughout the Roman Empire by Greek-speaking Jews. In the fourth century, when the Christian Bible was authorized in Greek as the Christian canon of scripture, the Old Testament included the books of the Septuagint.
Translators of the Hebrew scriptures into Greek or English must consider how to render a Semitic language into an Indo-European language, which is not always straightforward. In addition, there are variant readings of the Hebrew scriptures. Before about 100 CE there did not exist a single standard version of the various Hebrew scrolls read as scripture by the Jews. It is misleading, therefore, to refer to the "canon" of the Jewish scriptures, or the Old Testament, before this date.
When Jewish scriptures were written on scrolls they did not have a set order, for no single scroll contained all the texts. However, the Prophets were read after the Torah and before the Writings. When the church in the fourth century created its canon, it changed this order by placing the Prophets after the Writings. In Christian scripture the story of God moves from the giving of the law to Israel, to psalms and reflections on the law, and then to the prophetic witness that judges Israel and hopes for renewal of the covenant.
At the time of the Reformation Protestant translators of the Christian Bible used the Hebrew canon established about 100 CE for the Old Testament, rather than the Septuagint. This is why Protestant Bibles do not contain all the Old Testament books that are in Roman Catholic Bibles, which based the Old Testament on the books included in the Septuagint read by first century Jews and Christians. Protestants refer to the books in the Septuagint, which are excluded from the Old Testament in Protestant Bibles, as the Apocrypha.