A training aid for playing a stringed instrument of the type having a neck and a fingerboard for assisting an instrument player in the proper placement of the thumb on the back of the instrument neck while playing the instrument, said training aid comprising: (a) a solid piece positioned on the back of the neck of the instrument against which said instrument player's thumb will be positioned; (b) an attachment to the solid piece, wherein said attachment enables said solid piece to be added to the back of the instrument's neck or removed from the back of the instrument's neck. The instrument may be a guitar.