Generally, guitar has three main parts; they are head, neck and body.
There are tuning keys, nut, and fret in the head of guitar.
The function of those screws is for tuning the guitar strings.
In chord writing, the nut described similar to fret, but it has a thicker line.
Fret is a thin metal that can be found all along the fingering board.
The space among those fret are columns known as and Roman numeral marks it.