The Tercio was a Spanish formation using pikemen and musketeers, how would the musketeers being positioned? At the flank of the pikemen? In front of the pikemen?
This was answered pretty comprehensively here:
https://www.reddit.com/r/AskHistorians/comments/450aqa/why_was_the_spanish_tercio_unit_effective_in/
By /u/itsalrightwithme. Not to discourage other answers, of course.