Hreflang Tags Without a Reciprocal Will Be Ignored
Hreflang tags which are not reciprocated will be ignored, because Google assume it’s incorrect.
Hreflanged Pages in the Same Language Don’t Need to Be Duplicates
If you connect pages together with hreflang which are in the same language e.g. UK/US/Australia, they don’t need to be duplicates. They are allowed quite a lot of variation as if they were different languages.
Hreflang Require Reciprocal Links
Hreflang works at a page level. If any pages don’t have recriprocal hreflang tags, they’ll be ignored, but it won’t affect any other pages with the correct configuration
A Single Page Can Serve Multiple Hreflang Combinations
When setting international targeting with an hreflang tag, you can add multiple hreflangs to the same page. So a single page could have both an en-us, en-uk and x-default hreflang values.
Unwanted Sitelinks Pages Can Be Nonidexed
Sitelinks which do not disappear after being demoted, can be removed by adding a noindex. If the Sitelinks are showing incorrect international pages, hreflang tags can be used to identify the preferred version.
Hreflang URLs Should Always Be Canonical URLs
Don’t include any URLs that redirect, are non-indexable, canonicalised, otherwise they might be ignored.
Hreflang Should Canonicalise to Preferred HTTP/HTTPS Variation
When you have multiple language sites with hreflang, and you have http and https versions of the sites, you don’t need to worry about the hreflang for the non-canonical version. So if you canonicalise from http to https, then you don’t need any hreflang on the http.