UnDeclareNonCommutative[a, b, ...] undeclares a,b, ... to be noncommutative, i.e., DataType[a,b, ..., NonCommutative] is set to False.
Overview, DataType, DeclareNonCommutative.
DeclareNonCommutative[x]As a side-effect of DeclareNonCommutative x is declared to be of DataType NonCommutative.
DataType[x, NonCommutative]\text{True}
The inverse operation is UnDeclareNonCommutative.
UnDeclareNonCommutative[x]
DataType[x, NonCommutative]\text{False}
DeclareNonCommutative[y, z]
DataType[y, z, NonCommutative]\{\text{True},\text{True}\}
UnDeclareNonCommutative[y, z]
DataType[y, z, NonCommutative]\{\text{False},\text{False}\}