GCE Advanced Level
Most people study three A-Levels, and one AS-Level.  An AS-Level is the first half of an A-Level.  An AS-Level can be studied on its own, but to get an A-Level the second half must also be completed.  The second half of an A-Level is an A2-Level. A student might opt to appear for his AS (Advanced Subsidiary) and A2 together in one go.