There are 3 different series:

• Series LL:
  for compressed air, hydraulic return lines, propane, oil heating, automotive engineering, small refrigerating plants, apparatus construction, plastic lines etc.
  
• Series L:
  like series LL but for greater mechanical stresses, compressed air lines etc.
  
• Series S:
  heavy mechanical engineering, shipbuilding, mining, chemical industry, large refrigerating plants, pressure lines etc.
  

Determining the thread type of metric connections:

d2   = Outer thread diameter
d1   = union nut internal thread diameter
NW = nominal width
SW  = key width

Determining the thread type of inch couplings:

First measure the external thread diameter d2 or the internal thread diameter of the union nut d1. Then establish the pitch or threads per inch (= 25.3 mm). You can then determine the thread type based on the tables:

Withworth »Standard«

american