What do the following phrases have in common?

1. BLUE LAW
2. AUDIO GUIDE
3. WORK HISTORY
4. PHONE RECORD

Answer:

Both words in each phrase can be followed by BOOK to make a common phrase:

1. BLUE BOOK, LAW BOOK
2. AUDIO BOOK, GUIDE BOOK
3. WORK BOOK, HISTORY BOOK
4. PHONE BOOK, RECORD BOOK