 Arms 1 and 2 are sometimes called the "side" or "collinear" arms.Port 3 is called the "H-plane port", and is also called the "Σ port", "sum port" or the "P-port" (for "parallel").Port 4 is the "E-plane port", and is also called the "Δ port", "difference port", or "S-port" (for "series").