Getting the current interface number

When loading the interface, the WoW client skips over any addons whose interface version does not match its own. This is an effort to keep the client's behavior free of errors due to changes in the API. Each addon specifies (in its .toc file) the interface version with which it complies. So an addon's .toc file most likely contains a line similar to: Here, 20400 would mean that the addon is designed against the Patch 2.4 client (and any subsequent minor releases).
 * 1) Interface: 20400

There are a number of ways to get the current interface version:
 * It probably is : But this number is maintained locally, so it might be out of date.
 * Steal it from another addon : Recently updated addons, which are not listed as "out of date" by the client contain the latest Interface version in their toc tag.
 * Use : In particular,   should output the correct version to your chat frame.
 * Extract FrameXML and check FrameXML.toc: You can download Blizzard's User Interface Customization tool or Blizzard Support Site (new Style), use it to extract interface data, and peek at
 * View FrameXML.toc online: For instance at [ wowcompares].