Canonical URL

The same page can often be reached at several addresses: with and without www, http and https, with tracking or filter parameters. Search engines pick one as the canonical and mostly show that one.

You can tell Google which you prefer with a rel="canonical" link on the page, with redirects from the duplicates, and by linking consistently to one version. Google treats your choice as a strong hint, not a command.

Wrong canonicals are a common reason a page doesn't show: if a page points its canonical at another page, Google may drop it from the index.

In practice

In practice, pick one form of your address (with or without www, always https), redirect the other forms to it, and make sure every page's canonical tag points to its own preferred address. Watch for templates that set every page's canonical to the home page, a common mistake that can drop pages from the index.