rohdem:And why can't I change balls, at least between holes?
There is nothing in the Rules of Golf that prevents a golfer from
switching to a different brand of golf ball (i.e., from a Titleist to a
Bridgestone) on every hole on the course - so long as the change is made
between the play of holes.
However, there is something in the Rules of Golf that says a
tournament committee can impose such a rule.
It's called the "one ball condition" (in the rulebook, it's in Appendix
1, Part C), more commonly known as the "one ball rule." As you probably
know, all Tour events are played under the "one ball rule." And any
rules committee may adopt the "one ball rule" for its competitions.
The "one ball condition" requires the player to use the exact same brand
and type of ball throughout the round. For example, if you tee off the
first hole with a Titleist Pro V1x, then that's what you must play
throughout the round. You may not switch to any other brand of ball, nor
even to any other type of Titleist ball. You started with the Pro V1x,
so the Pro V1x is what you must use on every stroke.