The authority component is preceded by a double slash ("//") […] This is also why not all URIs contain the double slash: because not all URIs have an authority component (e.g., URIs using the mailto scheme, the xmpp scheme, etc.).

Why are there double slashes?

The slash is used as a division operator in most programming languages while APL uses it for reduction (fold) and compression (filter). The double slash is used by Rexx as a modulo operator, and Python (starting in version 2.2) uses a double slash for division which rounds (using floor) to an integer.

What does two backward slashes mean?

Double Backslashes (\\)

Two backslashes are used as a prefix to a server name (hostname). For example, \\a5\c\expenses is the path to the EXPENSES folder on the C: drive on server A5. See UNC, \\, path and forward slash.