Extract spec from https://github.com/mojombo/semver.org.

semver.md

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+Semantic Versioning 1.0.0
+=========================
+
+In the world of software management there exists a dread place called
+"dependency hell." The bigger your system grows and the more packages you
+integrate into your software, the more likely you are to find yourself, one
+day, in this pit of despair.
+
+In systems with many dependencies, releasing new package versions can quickly
+become a nightmare. If the dependency specifications are too tight, you are in
+danger of version lock (the inability to upgrade a package without having to
+release new versions of every dependent package). If dependencies are
+specified too loosely, you will inevitably be bitten by version promiscuity
+(assuming compatibility with more future versions than is reasonable).
+Dependency hell is where you are when version lock and/or version promiscuity
+prevent you from easily and safely moving your project forward.
+
+As a solution to this problem, I propose a simple set of rules and
+requirements that dictate how version numbers are assigned and incremented.
+For this system to work, you first need to declare a public API. This may
+consist of documentation or be enforced by the code itself. Regardless, it is
+important that this API be clear and precise. Once you identify your public
+API, you communicate changes to it with specific increments to your version
+number. Consider a version format of X.Y.Z (Major.Minor.Patch). Bug fixes not
+affecting the API increment the patch version, backwards compatible API
+additions/changes increment the minor version, and backwards incompatible API
+changes increment the major version.
+
+I call this system "Semantic Versioning." Under this scheme, version numbers
+and the way they change convey meaning about the underlying code and what has
+been modified from one version to the next.
+
+
+Semantic Versioning Specification (SemVer)
+------------------------------------------
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
+interpreted as described in RFC 2119.
+
+1. Software using Semantic Versioning MUST declare a public API. This API
+could be declared in the code itself or exist strictly in documentation.
+However it is done, it should be precise and comprehensive.
+
+1. A normal version number MUST take the form X.Y.Z where X, Y, and Z are
+integers. X is the major version, Y is the minor version, and Z is the patch
+version. Each element MUST increase numerically by increments of one. For
+instance: 1.9.0 -> 1.10.0 -> 1.11.0.
+
+1. When a major version number is incremented, the minor version and patch
+version MUST be reset to zero. When a minor version number is incremented, the
+patch version MUST be reset to zero. For instance: 1.1.3 -> 2.0.0 and 2.1.7 ->
+2.2.0.
+
+1. A pre-release version number MAY be denoted by appending an arbitrary
+string immediately following the patch version and a decimal point. The string
+MUST be comprised of only alphanumerics plus dash [0-9A-Za-z-] and MUST begin
+with an alpha character [A-Za-z]. Pre-release versions satisfy but have a
+lower precedence than the associated normal version. Precedence SHOULD be
+determined by lexicographic ASCII sort order. For instance: 1.0.0.alpha1 <
+1.0.0.beta1 < 1.0.0.beta2 < 1.0.0.rc1 < 1.0.0.
+
+1. Once a versioned package has been released, the contents of that version
+MUST NOT be modified. Any modifications must be released as a new version.
+
+1. Major version zero (0.y.z) is for initial development. Anything may change
+at any time. The public API should not be considered stable.
+
+1. Version 1.0.0 defines the public API. The way in which the version number
+is incremented after this release is dependent on this public API and how it
+changes.
+
+1. Patch version Z (x.y.Z | x > 0) MUST be incremented if only backwards
+compatible bug fixes are introduced. A bug fix is defined as an internal
+change that fixes incorrect behavior.
+
+1. Minor version Y (x.Y.z | x > 0) MUST be incremented if new, backwards
+compatible functionality is introduced to the public API. It MAY be
+incremented if substantial new functionality or improvements are introduced
+within the private code. It MAY include patch level changes.
+
+1. Major version X (X.y.z | X > 0) MUST be incremented if any backwards
+incompatible changes are introduced to the public API. It MAY include minor
+and patch level changes.
+
+
+Tagging Specification (SemVerTag)
+---------------------------------
+
+This sub-specification SHOULD be used if you use a version control system
+(Git, Mercurial, SVN, etc) to store your code. Using this system allows
+automated tools to inspect your package and determine SemVer compliance and
+released versions.
+
+1. When tagging releases in a version control system, the tag for a version
+MUST be "vX.Y.Z" e.g. "v3.1.0".
+
+1. The first revision that introduces SemVer compliance SHOULD be tagged
+"semver". This allows pre-existing projects to assume compliance at any
+arbitrary point and for automated tools to discover this fact.
+
+
+Why Use Semantic Versioning?
+----------------------------
+
+This is not a new or revolutionary idea. In fact, you probably do something
+close to this already. The problem is that "close" isn't good enough. Without
+compliance to some sort of formal specification, version numbers are
+essentially useless for dependency management. By giving a name and clear
+definition to the above ideas, it becomes easy to communicate your intentions
+to the users of your software. Once these intentions are clear, flexible (but
+not too flexible) dependency specifications can finally be made.
+
+A simple example will demonstrate how Semantic Versioning can make dependency
+hell a thing of the past. Consider a library called "Firetruck." It requires a
+Semantically Versioned package named "Ladder." At the time that Firetruck is
+created, Ladder is at version 3.1.0. Since Firetruck uses some functionality
+that was first introduced in 3.1.0, you can safely specify the Ladder
+dependency as greater than or equal to 3.1.0 but less than 4.0.0. Now, when
+Ladder version 3.1.1 and 3.2.0 become available, you can release them to your
+package management system and know that they will be compatible with existing
+dependent software.
+
+As a responsible developer you will, of course, want to verify that any
+package upgrades function as advertised. The real world is a messy place;
+there's nothing we can do about that but be vigilant. What you can do is let
+Semantic Versioning provide you with a sane way to release and upgrade
+packages without having to roll new versions of dependent packages, saving you
+time and hassle.
+
+If all of this sounds desirable, all you need to do to start using Semantic
+Versioning is to declare that you are doing so and then follow the rules. Link
+to this website from your README so others know the rules and can benefit from
+them.
+
+
+FAQ
+---
+
+### How do I know when to release 1.0.0?
+
+If your software is being used in production, it should probably already be
+1.0.0. If you have a stable API on which users have come to depend, you should
+be 1.0.0. If you're worrying a lot about backwards compatibility, you should
+probably already be 1.0.0.
+
+### Doesn't this discourage rapid development and fast iteration?
+
+Major version zero is all about rapid development. If you're changing the API
+every day you should either still be in version 0.x.x or on a separate
+development branch working on the next major version.
+
+### If even the tiniest backwards incompatible changes to the public API require a major version bump, won't I end up at version 42.0.0 very rapidly?
+
+This is a question of responsible development and foresight. Incompatible
+changes should not be introduced lightly to software that has a lot of
+dependent code. The cost that must be incurred to upgrade can be significant.
+Having to bump major versions to release incompatible changes means you'll
+think through the impact of your changes, and evaluate the cost/benefit ratio
+involved.
+
+### Documenting the entire public API is too much work!
+
+It is your responsibility as a professional developer to properly document
+software that is intended for use by others. Managing software complexity is a
+hugely important part of keeping a project efficient, and that's hard to do if
+nobody knows how to use your software, or what methods are safe to call. In
+the long run, Semantic Versioning, and the insistence on a well defined public
+API can keep everyone and everything running smoothly.
+
+### What do I do if I accidentally release a backwards incompatible change as a minor version?
+
+As soon as you realize that you've broken the Semantic Versioning spec, fix
+the problem and release a new minor version that corrects the problem and
+restores backwards compatibility. Remember, it is unacceptable to modify
+versioned releases, even under this circumstance. If it's appropriate,
+document the offending version and inform your users of the problem so that
+they are aware of the offending version.
+
+### What should I do if I update my own dependencies without changing the public API?
+
+That would be considered compatible since it does not affect the public API.
+Software that explicitly depends on the same dependencies as your package
+should have their own dependency specifications and the author will notice any
+conflicts. Determining whether the change is a patch level or minor level
+modification depends on whether you updated your dependencies in order to fix
+a bug or introduce new functionality. I would usually expect additional code
+for the latter instance, in which case it's obviously a minor level increment.
+
+
+About
+-----
+
+The Semantic Versioning specification is authored by [Tom Preston-Werner](http://tom.preston-werner.com), inventor of Gravatars and cofounder of GitHub.
+
+If you'd like to leave feedback, please [open an issue on GitHub](https://github.com/mojombo/semver.org/issues).
+
+
+License
+-------
+
+Creative Commons - CC BY 3.0
+http://creativecommons.org/licenses/by/3.0/