As of OTP release 17, the OTP release number corresponds to the major OTP version number. In the normal case, the OTP version number will be constructed as <Major>.<Minor>.<Patch>. However more dot separated parts may exist. If all parts less significant than Minor equals 0, they are omitted when printing the version. Release candidates have an -rc<N> suffix. The OTP version as a concept was introduced in OTP 17.

OTP of a specific version is a set of applications of specific versions. The application versions identified by an OTP version corresponds to application versions that have been tested together by the Erlang/OTP team at Ericsson AB. An OTP system can however be put together with applications from different OTP versions. Such a combination of application versions has not been tested by the Erlang/OTP team. It is therefore always preferred to use OTP applications from one single OTP version.

In an OTP source code tree as well as in an installed OTP development system, the OTP version can be read from the text file OTP_VERSION in the OTP installation root directory (code:root_dir()).

If the version read from the OTP_VERSION file in a development system has a ** suffix, the system has been patched using the $ERL_TOP/otp_build patch_app tool. In this case, the system consists of application versions from multiple OTP versions. The version preceding the ** suffix corresponds to the OTP version of the base system that has been patched.

On a target system (see the system principles documentation) no OTP_VERSION file will exist. This since one easily can create a target system where it is hard to even determine the base OTP version.

Note that if a development system is updated by other means than $ERL_TOP/otp_build patch_app, the OTP_VERSION file may identify wrong OTP version.