The unit-derived method in coding theory is shown to be a unique optimal
scheme for constructing and analysing codes. In many cases efficient and
practical decoding methods are produced. Codes with efficient decoding
algorithms at maximal distances possible are derived from unit schemes. In
particular unit-derived codes from Vandermonde or Fourier matrices are
particularly commendable giving rise to mds codes of varying rates with
practical and efficient decoding algorithms. For a given rate and given error
correction capability, explicit codes with efficient error correcting
algorithms are designed to these specifications. An explicit constructive proof
with an efficient decoding algorithm is given for Shannon's theorem. For a
given finite field, codes are constructed which are `optimal' for this field