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The basic unit of named data in a netCDF file is a variable. In general, a variable is a multidimensional object that has, among other characteristics, a shape, which is defined by the number, order, and sizes of its dimensions. When a netCDF variable is defined, the number and order of the dimensions that define its shape are specified. Hence you must first create the necessary dimensions before creating a netCDF variable that uses them.
It is possible to use the same dimension more than once in specifying a
variable shape, for example var(dim, dim), but it does not make
much sense to do this; it is contrary to the intuitive meaning of a
physical dimension. A variable that has two dimensions that happen to
be the same size is more accurately modeled by using two dimensions with
different names but the same size.
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