Musical Symbols: What do those Roman Numerals in music such as IV and V7 mean?

You don't see them much anymore except in music theory books. But Roman Numerals used to be used extensively to show what chord is being used in a piece of music, and what inversion of the chord is to be used. They were musical symbols, much like chord symbols are today.

Here is a chart showing the relationship of Roman numerals used in music to classical definitions:


I Tonic

V Dominant

IV Sub-Dominant

ii Super-Tonic

iii Mediant

vi Sub-Mediant

vii Sub-Tonic

And by the way, that is the order of likelihood as well -- the Tonic (I -- the home base of any key) is the most used and most likely chord in any key. The Dominant (V) is the next most used and most likely chord, followed by the Sub-Dominant. Those are the "Big 3" -- also known a the "primary chords" in any key.

After the primary chords, the next most used chord is the ii chord (notice the lower case Roman numerals -- that indicates that the chord in it's natural, organic state, is minor. Next comes the iii, followed by the vi, with the vii bringing up the rear.

Inversions are shown by an Arabic number following the Roman Numeral, such as  I 6/4 , or V 7, or IV 6.
Roman Numerals without an Arabic number following it would assumed to be root position chords. Those with an Arabic "6/4" would mean 2nd inversion of the chord, while an Arabic "6" following the Roman Numeral would indicate 1st inversion. A "7" after a Roman Numeral would mean to add a 7th to a chord.