I’m trying to wrap my mind around the concept of Master Pages. What I’m currently not understanding is the following: When I set up the “First GOOD” Master Page Default Part and double-click on it, I get the following (just as I expect):
Did you apply your master page to page 1?
The Master Page Editor is showing you a left page, which you then need to copy to the right page. There’s a button that will do this for you, to the left of the “Apply” button at the top of the window.
Page 1 of your viola part is a right page.
D.D., in addition to Dan’s and pianoleo’s solution a small hint for even better productivity:
If you add text frames, where you later wish the text to be centred: do not mind spanning the frame across the whole page - as afterwards it will be centred 
See this example:
Thanks all - very helpful (definitely hadn’t previously occurred to me that the first page is considered a “right-most” page and that the default had only applied to the left - probably because it’s a part and so I wasn’t thinking in terms of normal, two-sided page turns - as the pages will likely be taped edge to edge, one-side only)…Also definitely handy suggestion to make any text boxes the full page width and then to set the text to center to ensure it’s absolutely centered (!)…
