Sentence:  However , the possible absence of a center of symmetry not only moves the hydrogen atom off **f , but also allows the oxygen atoms to become nonequivalent , with **f at **f and **f at **f ( space group **f ) , where **f represents the oxygens on one side of the **f <WSD>layers</WSD> and **f those on the other side . 
Meaning of the word marked with <WSD> tags:  single thickness of usually some homogeneous substance