“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” second intervals:

Major Seconds: do-re, re-mi, fa-so, so-la, te-do (and their descending counterpart)

Minor Seconds: mi-fa, la-te

Level 1

Level 2

Level 3