Is there a standard naming convention for git tags?

Version 1.0.0 of Semantic Versioning, by Tom Preston-Werner of GitHub fame, had a sub-specification addressing this:

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".

However, after discussion this was removed, and is no longer present in the latest version of the SemVer spec (2.0.0 at the time of writing). A later discussion thread in the same place went into greater depth, and resulted in a new Is "v1.2.3" a semantic version? being added to the FAQ in SemVer's master branch, although at the time of writing (over 2 years later) this change is still not present in the officially released spec.

Not that I know of.
But Git will not allow a tag and a branch of the same name at the same time, so if you have a branch "1.1" for 1.1 works, do not put a tag "1.1", use for instance "v1.1"

We use branches and tags for release-specific work followed by the actual release, respectively:

o---o-----o---o---o--- ...   master
     \   /       /
      \ /       /
       o-------o--- ...      1.6 branch

Every developer makes a mental decision about whether the work they're about to commit is applicable just to master or if it's also relevant to the branch. You can see that changes that are made to the branch are merged back on master, but some changes on master will never go on the branch (that is, those not intended for the 1.6 release, in this example).

When we're ready to release, we tag it and then merge back one last time, and we name the tag with the same name as the branch, but with an extra identifier about what particular version it is, e.g. "1.6-release" or "1.6-beta" or "1.6-rc2", et cetera.

... ------o---o---o--o---o--- ...   master
         /       /
        /       /
... ---o------(*)--- ...      1.6 branch
          1.6-release

I don't know of any standards. I simply choose my tag names such that I can stick a

VERSION = `git describe --tags`

in my build scripts. So, the tag naming convention actually depends on the version naming convention of the project.

There is no one best practice I'm aware of. Here are some links:

• http://web.elctech.com/?p=79
• http://reinh.com/blog/2009/03/02/a-git-workflow-for-agile-teams.html#production-tagging

Generally, versioning (0.0.1, v0.2.1, ...) maybe hand in hand with some issue-tracking could be considered a plausible approach. (.. although I usually use v-prefixed tag names .. see also @VonC answer)