Codex Alexandrinus is one of the four Great uncial codices and among the earliest and most complete manuscripts of the Bible. It is the oldest manuscript to use larger letters to indicate new sections, and it was the first manuscript of great importance and antiquity to be extensively used by textual critics.

The text is uniquely written partly in columns and partly in a cruciform (cross-shaped) layout. It includes full marginal notes, contains the pericope adulterae, and marks John 5:3-4 with an obelus to indicate it is doubtful.