C# Definition:

/// <summary>
/// Selects duplex or double-sided printing for printers capable of duplex printing.
/// </summary>
internal enum DMDUP : short
{
   /// <summary>
   /// Unknown setting.
   /// </summary>
   DMDUP_UNKNOWN = 0,

   /// <summary>
   /// Normal (nonduplex) printing.
   /// </summary>
   DMDUP_SIMPLEX = 1,

   /// <summary>
   /// Short-edge binding, that is, the long edge of the page is horizontal.
   /// </summary>
   DMDUP_VERTICAL = 2,

   /// <summary>
   /// Long-edge binding, that is, the long edge of the page is vertical.
   /// </summary>
   DMDUP_HORIZONTAL = 3,
}

Notes:

The definition of DMDUP_UNKNOWN is only to satisfy the rule, that every enum should have a zero value.