SD– The guitar neck that points upwards is the subdominant. It stretches out of the mid portion of the guitar body. That makes it easy for one to grip the neck firmly and bring it around wherever you go.
SO– That would be the “headstock”. It’s the part where you see those 6 “switches” for the guitar strings, and you can twist and turn the switches as and when you like it.
Another SO could be the sound hole. It’s the circular hole in the body. What make it the SO is the fact that it gives contrast to the large surface surrounding it. Also, since it’s a hole, it works in giving the guitar sound an echo.