Quote Originally Posted by ark View Post
Aha! I see the problem. If a bar has "D.S. al 3rd end" but ends with a single rather than a double barline, the instruction seems to be ignored during playback.

So there's something magic about double barlines. Can you explain the rules for how the player treats barlines? Also, it seems to me that it is possible for two bars to be separated by two barlines, which you would do by putting one after the first bar and another before the second. That seems weird for me, and I would think there is no reason ever to do it; but perhaps you could tell me if that ability is intended to be useful and, if so, what its effect is?
Yes you need a double bar for a rehearsal instruction to work.
This makes sense as you not would jump D.C. from the middle of a section (the rest of that section would then never be played)

You have left bar lines and right bar lines.
You can use whichever you like.
If you are at the end of a system you need the right and if you are at the beginning you use the left.