𝐴 is 𝑚×𝑛 Definition 𝐴𝑇𝐴=𝐼 and 𝐴𝐴𝑇=𝐼. det(𝐴)=±1 𝐴−1=𝐴𝑇 columns of Orthogonal matrix are mutually orthogonal 𝐴𝑇 is Orthogonal matrix