Nick got it right...until the end.
Regular 6 strings guitar can be tuned down to drop B, even though most of them only tune it down to drop C which tunes the strings to CGCFAD, drop D is DADGBE.
However to tune down drop A and drop F, you would need a 7 string guitar for Drop A. Which is lower than drop C by a minor third. Im not sure if you can tune down a 7 strings to drop F since that would be, an augmented 5th tune down from B, which is a big drop, but I know 8 strings guitar can be tuned down to it, which would be lower than drop A.
so in the musical Alphabet, notes repeat and they go ABCDEFG. you start with E, and dropping it down would go down the notes, which tells you the order of lower would be E standard, Drop D, Drop C, Drop B(or 7th string standard), Drop A, Drop G, F#(8 strings standard), and drop F. Ofcourse you can drop to the notes in between as well, Eb/D#, Db/C# etc.