There are several types fittings and unions used on hot tubs and spas.
below is a breif description of some of the types and abbreviations used.

• PIPE FITTINGS
  • BARB type fittings are used with flexible tubing and clamps and glue. Barbs can be smooth or ribbed.
  • FPT means female pipe thread
  • HOSE ADAPTER, means the end of a vacuumn hose will fit over this. used mainly on pools.
  • MPT mean male pipe thread.
  • SLIP means pipe will glue inside of this fitting.
  • SPG or SPIGOT, means a slip fiitting will glue over this and it is the same size as pipe.
  • SLIP (SPG) COMBINATION - Pipe will glue inside this and fitting of larger size will glue over it.

• UNION THREADS
  • Most all union nut threads are buttress threads.
  • However the pilot thread depth will vary and effect compatibility. See picture below.