AMP URLs

It's worth noting that a side-effect of the AMP Cache is that AMP documents will have a separate URL for the cached version. In fact, AMP documents have three different kinds of URL. For domain example.com, these will be:

• Original URL:
  https://example.com/amp_doc.html

• AMP Cache URL:
  https://example-com.cdn.ampproject.org/c/s/example.com/amp_doc.html

• AMP Viewer URL (when viewing AMP pages from Google search results):
  https://www.google.com/amp/example.com/amp_doc.html

Having three URLs might be a little confusing, but they're necessary for caching and pre-rendering in AMP.