It is shown that a subharmonic grating may have two different phase re
lationships relative to the fundamental grating, leading to the appear
ance of domains. The subharmonic beam is generated in a Bi12GiO20 crys
tal illuminated by two beams in the presence of an applied ac square-w
ave voltage. The domain structure and the relative phases of the grati
ngs contained in them have been observed with the aid of an interferom
etric technique.