The way the guitar neck is shaped allows your left hand to fit comfortably around the neck. This will help your acoustic guitar playing a lot almost by accident. You need to pay attention to where you place your thumb. It can go on the back of the neck or along the side. It depends on what type of playing you are doing. If you are playing single strings it may feel best if you place it along the side of the neck. On the other hand some types of chords require your thumb to be more on the back of the neck.