By applying the formulas for seventh chords to the C major scale, the following series of chords is created. These are called scale tone seventh chords. If you analyze the notes of these scale tone 7th chords, you will notice that they are all based on the C major scale tone triads and each one has another 3rd interval added above it.
It is essential to be able to play these scale tone 7th chords as arpeggios in all five forms on the fretboard, and in all twelve keys. Here is an example in C major. Learn it from memory in the open position and then work out the other fingerings as well as transposing it to all the other keys. Don’t be scared off by the absence of TAB here, all these notes can be found in the open position (i.e. the open strings and the first 3 frets).
When numbering any types of chords within a key, Roman numerals are used. This makes it clear that a chord is being described rather than a single note (scale degree) which would be indicated by a number (1, 2, b3, #4 etc).