  We discuss relations between closed and open string amplitudes at one-loop.
While at tree-level these relations are known as Kawai-Lewellen-Tye (KLT)
and/or double copy relations, here we investigate how such relations are
manifested at one-loop. While there exist examples of one-loop closed string
amplitudes that can strikingly be written as sum over squares of one-loop open
string amplitudes, generically the one-loop closed string amplitudes assume a
form reminiscent from the one-loop doubly copy structure of gravitational
amplitudes involving a loop momentum. This double copy structure represents the
one-loop generalization of the KLT relations.
