How to Properly Write a URL

A common mistake that many people make

Yihui Xie 2018-01-30

My two least favorite parts to write in the blogdown book were: the chapter on Hugo (Chapter 2), and the appendices (B and C) on some basic knowledge about web technologies. I found it quite challenging to describe the full pictures, and some technical details were frustrating for me to introduce. I’ll give a simple example in this post: how to write a valid URL (or link).

You may think it is dead simple to write a URL, and we click links to browse websites every day. We see them in the address bar of our web browsers all the time.

You are wrong. People make a common mistake all the time when writing URLs without realizing the mistake. I’m not kidding. If you are familiar with Markdown, would you think the following link is valid?

Please visit the [R project homepage](

Or if you know HTML, does the following HTML code produce a valid link?

Please visit the <a href="">R project homepage</a>.

They may appear to be correct at the first glance. However, these links won’t take your readers to the R project website. Why? Because they are relative links. Carefully read Appendix B.1 in the blogdown book to torture yourself by the gory details about “relative URLs”, “absolute URLs”, “relative absolute URLs”, and “protocol-free URLs”, etc.

This is a valid link:

Please visit the [R project homepage](

And this is also valid when present on an HTML page served through a protocol (HTTP or HTTPS):

Please visit the [R project homepage](//

Just don’t write a link like this:

Please visit the [R project homepage](

It is wrong, wrong, wrong.