What is the semantics of the double slash following the scheme in a URI?

Question

According to http://tools.ietf.org/html/rfc3986 and http://en.wikipedia.org/wiki/Uniform_resource_identifier, a URI may or may not contain a double slash following the schem

Answer 1

It's in the RFC you linked: If there is a //, it means that what follows that is the authority. See Section 3. So if the scheme uses an authority, it will use the // after the colon (either requiring it, if authority is required in that scheme, or having it be optional if authority is optional in that scheme). mailto doesn't use an authority in the URI sense, so mailto URIs don't include a //.