## unitary matrix question

"orthonormal columns imply orthonormal rows for square matrix."

My proof is:
$Q^{T}Q=I$(orthonormal columns)
implys
$QQ^{T}=I$(orthonoraml rows)

for square matrix.

But i think this proof is kind of indirect. Is there another more direct proof from the definition of inner product or norm?
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 Mentor I think that's as direct and simple as it gets. I would only add the explanation of how $Q^TQ=I$ implies $QQ^T=I$.