diff --git a/src/types.jl b/src/types.jl
index 34b58362..fbb68e1d 100644
--- a/src/types.jl
+++ b/src/types.jl
@@ -1,4 +1,7 @@
 # All types using in `NCDatasets`
+# Note in CommonDataModel the special properties `attrib`, `dim` and `group`
+# are made available.
+
 
 # Exception type for error thrown by the NetCDF library
 mutable struct NetCDFError <: Exception
@@ -6,32 +9,18 @@ mutable struct NetCDFError <: Exception
     msg::String
 end
 
-# base type of attributes list
-
-abstract type BaseAttributes
-end
-
 abstract type AbstractNCDataset <: AbstractDataset
 end
 
 abstract type AbstractNCVariable{T,N} <: AbstractVariable{T,N}
 end
 
-
-abstract type AbstractDimensions
-end
-
-abstract type AbstractGroups
-end
-
-
 # Variable (as stored in NetCDF file, without using
 # add_offset, scale_factor and _FillValue)
 mutable struct Variable{NetCDFType,N,TDS} <: AbstractNCVariable{NetCDFType, N}
     ds::TDS
     varid::Cint
     dimids::NTuple{N,Cint}
-#    attrib::Attributes{TDS}
 end
 
 mutable struct NCDataset{TDS} <: AbstractNCDataset where TDS <: Union{AbstractNCDataset,Nothing}
@@ -130,7 +119,6 @@ end
 
 # view of subsets
 
-
 struct SubVariable{T,N,TA,TI,TAttrib,TV} <: AbstractNCVariable{T,N}
     parent::TA
     indices::TI
diff --git a/src/variable.jl b/src/variable.jl
index cd44cb18..e613650c 100644
--- a/src/variable.jl
+++ b/src/variable.jl
@@ -222,8 +222,8 @@ end
 export dimnames
 
 
-function dim(v::AbstractNCVariable,dimname::AbstractString)
-    if !(dimname in dimnames(v))
+function dim(v::AbstractNCVariable,dimname::SymbolOrString)
+    if !(String(dimname) in dimnames(v))
         error("$dimname is not among the dimensions of $(name(v))")
     end
     return dim(dataset(v),dimname)