issues: 211888882

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
211888882 MDU6SXNzdWUyMTE4ODg4ODI= 1295 Terminology for the various coordinates 10050469 closed 0     8 2017-03-04T16:12:53Z 2017-03-15T16:28:12Z 2017-03-15T16:28:12Z MEMBER      

Picking up a thread about the repr (https://github.com/pydata/xarray/issues/1199#issuecomment-272824929), I think it would be good to give a name to the two different types of coordinates in xarray.

Currently the doc says:

One dimensional coordinates with a name equal to their sole dimension (marked by * when printing a dataset or data array) take on a special meaning in xarray. They are used for label based indexing and alignment, like the index found on a pandas DataFrame or Series. Indeed, these “dimension” coordinates use a pandas.Index internally to store their values.

Other than for indexing, xarray does not make any direct use of the values associated with coordinates. Coordinates with names not matching a dimension are not used for alignment or indexing, nor are they required to match when doing arithmetic (see Coordinates).

The use of quotation marks in “dimension” coordinates makes the term imprecise. Should we simply call the former dimension coordinates and the latter optional coordinates?

This would also help to uniformize error reporting (e.g. https://github.com/pydata/xarray/pull/1291#discussion_r104261803)

 
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