I asked this question last year of an assistant pro at my local pro shop–the head pro/director of golf there builds clubs himself but I didn’t ask him–as to whether or not the orientation of the flat side of “Flatso” type putter grips is constrained in any way according to the current rules of golf.  Can I align the flat side in any direction 360 degrees or all around the clock, or must the flat side of such a single flat designed grip be flat side “up?”

I’m aware there are square cross-section grips available, so it seems to me a grip can have a flat side toward each of the four directions simply because the square grip does by it’s sheer design…but if there is only ONE flat side, must it be oriented flat side “up?” As though the eyes look down on top of the flat top, with the shaft descending and the club face perpendicular to it?