“MOVABLE DO”: having the FA on the 3rd line is equivalent to having the DO on the ledger line above the 4-line staff .

Notice the difference of interval between the “Major” and “Minor” third intervals:

Major Thirds: do-mi, fa-la, so-ti (and their descending counterpart)

Minor Thirds: la-do, ti-re, re-fa, mi-so.

Level 1

Level 2

Level 3