Great question — answering as the UphillTech team:
www.example.com/abc will have these attributes: <link rel=“canonical” href=“example.com/abc” /> <link rel="alternate" href="example.com/abc" hreflang="en"> <link rel="alternate" href="example.com/en_us/abc" hreflang="en-US"> <link rel="alternate" href="example.com/abc" hreflang="x-default"> www.example.com/en_US/abc will have : <link rel=“canonical” href=“www.example.com/en_US/abc” /> <link rel="alternate" href="example.com/abc" hreflang="en"> <link rel="alternate" href="example.com/en_us/abc" hreflang="en-US"> <link rel="alternate" href="example.com/abc" hreflang="x-default"> note that the slashes at the end need to be added to all if your site using them else dont add the slashes at then...
Full answer by yahia ashraf: https://webmasters.stackexchange.com/a/142867 (CC BY-SA 4.0).
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