  Double Toeplitz (shortly DT) codes are introduced here as a generalization of
double circulant codes. We show that such a code is isodual, hence formally
self-dual. Self-dual DT codes are characterized as double circulant or double
negacirculant. Likewise, even DT binary codes are characterized as double
circulants. Numerical examples obtained by exhaustive search show that the
codes constructed have best-known minimum distance, up to one unit, amongst
formally self-dual codes, and sometimes improve on the known values. Over
$\F_4$ an explicit construction of DT codes, based on quadratic residues in a
prime field, performs equally well. We show that DT codes are asymptotically
good over $\F_q$. Specifically, we construct DT codes arbitrarily close to the
asymptotic varshamov-Gilbert bound for codes of rate one half.
