The Gray map from
to binary pairs is the bijection
,
,
, and
, extended coordinatewise to words
over
. It takes a length-
quaternary code to a length-
binary code.
Its binary image need not be a linear code, even when
the quaternary code is linear over the ring
.
The modified binary Preparata-like codes of Hammons et al. (1994) are Gray images of linear codes over .