Transpose
Transpose[mat]
transposes the first two levels of mat
Attributes[Transpose] := {Protected}
Simple examples
In[1]:= Transpose[{a, b}] Out[1]= Transpose[{a, b}]
In[2]:= Transpose[{{a}, {b}}] Out[2]= {{a, b}}
In[3]:= Transpose[{{a, b}}] Out[3]= {{a}, {b}}
In[4]:= Transpose[{{{a}, {b}}}] Out[4]= {{{a}}, {{b}}}
In[5]:= Transpose[a] Out[5]= Transpose[a]