Word Tag

Let's start with two words like POP MUSIC and the next poster might say MUSIC NOTE and so on. You tag on to the last word and make 2 more words from that (KEEPING THE SECOND WORD and USING IT AS THE FIRST ONE in your answer...). Note: Also, use only the form of the word(s) in your answers as they are given in the post ahead of yours - don't change singulars to plurals, etc. ~ part of the challenge of the game is to think forward, leaving a word that is usable in forming a *next* sensible answer.NOTE: *Sensible* meaning that the words can be used together, as written, in some sort of real sentence-[offered in hopes of clarifying the original intention of the game]

1. POP MUSIC
2. MUSIC NOTE
3. NOTE BOOK
4. BOOK INDEX