Source : Free On-Line Dictionary of Computing

horizontal encoding
     
         An {instruction set} where each field (a bit or
        group of bits) in an instruction word controls some
        {functional unit} or {gate} directly, as opposed to {vertical
        encoding} where instruction fields are decoded (by
        {hard-wired} {logic} or {microcode}) to produce the control
        signals.  Horizontal encoding allows all possible combinations
        of control signals (and therefore operations) to be expressed
        as instructions whereas vertical encoding uses a shorter
        instruction word but can only encode those combinations of
        operations built into the decoding logic.
     
        An {instruction set} may use a mixture of horizontal and
        vertical encoding within each instruction.  Because an
        architecture using horizontal encoding typically requires more
        instruction word bits it is sometimes known as a {very long
        instruction word} (VLIW) architecture.
     
        (1995-04-23)