Intra-class synthesis
Synthesis from two sections of the same main class achieved by + and omission of the main class letter
Cats and dogs = F952 + F918 = F952 + 918
No preferred citation order
Inter-class synthesis
Synthesis from two main classes by + and retention of the main class letter
Logic and rhetoric = A300 + M170
Gambling in dog racing = L933 + F944
Called “composite subjects” or “composite books”
No preferred citation order