Why are files in 16 hive referred through _layouts/15 on a SharePoint 2016 Server?

Question

I'm converting a SharePoint 2013 solution to 2016. Back when 2013 was introduced I recall all the _layouts/ -> _layouts/15 issues, and immediately looked out for this.

In my SharePoint 2016 solution there is an icon image on this location:

C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\TEMPLATE\IMAGES\FGIMG.PNG

I would expect the mapped url to be:

_layouts/16/images/fgimg.png

However that is NOT the case. The image is found here:

_layouts/15/images/fgimg.png

This confuses me.

Why is this so?

I can't seem to find any official documentation about this...

(It is a clean 2016 RTM install and that file is a part of SharePoint itself)

Answer 1

Alright, got answers elsewhere. Here is an excerpt:

A standard installation of SharePoint Server 2016 will only create a 14 folder and a 16 folder. There is no 15 folder in the hive (i.e. at ...\Web Server Extensions\15). The reason they did this was to not break compatibility with v15 sites.

Furthermore there's no compatibility level available in 2016 (yes the switch is there, but there's no point). All sites are v15 and there is no backwards compatibility with v14.5. This was for ease of migration.

This was essentially a feature for Microsoft and is why all sites must be in v15 compat mode prior to upgrading to 2016.

The _layouts/16 doesn't work because it doesn't exist nor does it need to exist. The virtual directory points _layouts/15 -> \Program Files\Common Files...\16. You can look at this in IIS.

Source: https://social.technet.microsoft.com/Forums/office/en-US/68547ab9-4c8f-423f-911f-ef926ad7a2e2/layouts-strategy-changed-again-in-sp2016?forum=SP2016