Codex Alexandrinus is a fifth-century Greek Bible, bound now in four volumes, written two columns to the page on parchment that has grown thin with age. In places the iron in the ink has eaten through the skin, so a letter written on one side shows on the other, and conservators have had to work around damage that the writing itself caused.
Its contents are generous. The complete Greek Old Testament is there, including the books that Protestant canons later set aside, followed by the New Testament and then the two Epistles of Clement, early Christian letters that no canon retained. Its own table of contents lists the Psalms of Solomon at the end, but those leaves are gone, so the index records a book the manuscript no longer holds.
The feature that has occupied scholars most is that its text is not consistent. In the Gospels it reads with the Byzantine tradition, the family that later became standard across the Greek-speaking church. In Acts, the Epistles and Revelation it reads with the Alexandrian, the family that Sinaiticus and Vaticanus represent. One book, two allegiances. The usual explanation is that its scribes copied from different exemplars for different parts, which is a plain enough answer and also a reminder that a manuscript is an assembly rather than a single act.