Hi, this is a theory lesson for the relative novice, The Major Scale and the Roman Numeral system are very important to understand as they provide a framework of understanding to build upon as the student advances.
Ok so lets start with the fact that each octave is sub-divided into 12 notes, or semi-tones that are represented on a guitar by frets :
1 note (of 12) = 1 semi-tone (of 12 ) = 1 fret ( of 12 ) so once you hit the 12th fret on any string on your guitar, the name of the note returns to the name of the same string played open , this is because the names of notes revolve and turn full circle at each octave or every 12 semitones. Now an "octave" is so called because it essentially contains 8 notes, out of the 12 notes mentioned above, these 8 notes are what make up :
THE DIATONIC MAJOR SCALE
The notes of the Major Scale as any scale does, start with the "ROOT" or " I " or 1st note of the scale, key of C used as example as it has no sharps or flats and is easier to observe the scale pattern.
e.g. KEY OF C :
POSITION : 1st 2nd 3rd 4th 5th 6th 7th 8th(octave)
NUMERAL : I II III IV V VI VII VIII
NOTE : C D E F G A B C
SOUND : Doh ray me fah so lah te doh
STEPS @ TONE OR 2 SEMITONES = 2 FRETS / OR 1 SEMITONE = 1 FRET
Whats critical to understand is the " Step Pattern " of the scale, that is the spaces between the notes, and the step pattern of the major scale is :
I -tone- II -tone- III -semi- IV -tone- V -tone- VI -tone- VII -semi- VIII
doh ray me fah so lah te doh
Some of the steps are Tone steps, some are semitone steps, and the Roman Numerals are used to help us understand these positions as we work in the different keys of which there are 12. The doh ray me sounds were introduced many moons ago to cut through language barriers in order to help folks understand the scale and its steps.
if C is I, D is II, E is III, F is IV etc
if G is I, A is II, B is III etc
The other important thing on this subject to remember is that with the exception of E-F and B-C all other notes have #s and ds (sharps and flats) between them, so with knowledge of the alphabet up to G, and this basic knowledge of The Major Scale Step pattern and roman numerals you are set up.