Major Scales:
Semi-tone/Half-step = the distance between 1 note to the next, i.e. C – Db or B-C etc.
Whole-step/whole-tone = the distance between a whole step i.e. C-D or B – C# etc.
The formula to creating a major scale is whole step, whole step, semi-tone, whole step, whole step, whole step, semi-tone. For example, the C major scale is really easy because it’s just all the white notes from C to C in octave but, if you wanted to work out the G Major scale then it would be G, A, B, C, D, E, F#, G.
Once you’ve wrote out a scale, you can easily find all the chords by simply taking a note, missing one, taking one, missing on and taking one, i.e. in the C major the scale is C D E F G A B C so an F chord from this scale would be F A C because you took F, missed out G to take A and then missed B to take C.
Using Roman Numerals to label the chords can tell you if they’re major or minor chords, i.e. C would be I but because it’s a capital letter it’s a major sound. On the other side, D is ii so it’s a minor sounding chord. The Roman Numerals concept is always the same so I- always major, ii- always minor, iii- always minor, IV- always major, V- always major, vi- always minor, viii- always diminished and then it ends with the root note again.
When writing a song, there are certain combinations that work best and are commonly used. For example, 1,4,5 (Roman Numerals) are a very common formula, sometimes with 6 too, and it just gives the song a pleasant sound that is very appealing to the general public. Although, look what notes are in what chord to evaluate how much of a sound change you’re going to get because changing from C major chord (C,E,G) to E major chord (E,G,B) won’t give you much of a different sound because they’re pretty much the same chord but a B instead of a C and in a different order. Typical chord changes are usually 2-5 and 3-4-6. However, you don’t ever really want 6-2 because it sounds unfinished.
Usually 1 is used as a “home” to finish the song so if you’re not wanting to use a 1 then you tend to stick to the pattern of 1,4,5.