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+ABI - Application Binary Interface
+==================================
+
+ "ABI" describes the low-level interface between an application program and a
+ library. Calling conventions, function arguments, return values, struct
+ sizes/defines and more.
+
+ [Wikipedia has a longer description](https://en.wikipedia.org/wiki/Application_binary_interface)
+
+## Upgrades
+
+ In the vast majority of all cases, a typical libcurl upgrade does not break
+ the ABI at all. Your application can remain using libcurl just as before,
+ only with less bugs and possibly with added new features. You need to read
+ the release notes, and if they mention an ABI break/soname bump, you may have
+ to verify that your application still builds fine and uses libcurl as it now
+ is defined to work.
+
+## Version Numbers
+
+ In libcurl land, you really can't tell by the libcurl version number if that
+ libcurl is binary compatible or not with another libcurl version.
+
+## Soname Bumps
+
+ Whenever there are changes done to the library that will cause an ABI
+ breakage, that may require your application to get attention or possibly be
+ changed to adhere to new things, we will bump the soname. Then the library
+ will get a different output name and thus can in fact be installed in
+ parallel with an older installed lib (on most systems). Thus, old
+ applications built against the previous ABI version will remain working and
+ using the older lib, while newer applications build and use the newer one.
+
+ During the first seven years of libcurl releases, there have only been four
+ ABI breakages.
+
+ We are determined to bump the SONAME as rarely as possible.  Ideally, we
+ never do it again.
+
+## Downgrades
+
+ Going to an older libcurl version from one you're currently using can be a
+ tricky thing. Mostly we add features and options to newer libcurls as that
+ won't break ABI or hamper existing applications. This has the implication
+ that going backwards may get you in a situation where you pick a libcurl that
+ doesn't support the options your application needs. Or possibly you even
+ downgrade so far so you cross an ABI break border and thus a different
+ soname, and then your application may need to adapt to the modified ABI.
+
+## History
+
+ The previous major library soname number bumps (breaking backwards
+ compatibility) have happened the following times:
+
+ 0 - libcurl 7.1,   August 2000
+
+ 1 - libcurl 7.5    December 2000
+
+ 2 - libcurl 7.7    March 2001
+
+ 3 - libcurl 7.12.0 June 2004
+
+ 4 - libcurl 7.16.0 October 2006