For example, Em(onD) and Em(onC♯). I found these in a Japanese guitar tab book I bought.
It's probably a shorthand for slash notation. Try putting the "on" note in the bass below whatever other chord is notated.
As @NReilingh says, it's probably a Japanese alternative to "slash notation".
"C/B" for example -- often read aloud as "C over B" means a C chord played over a prominent B bass note.
The easy way to play these is to have a bassist! You play a normal C, the bassist plays the B.
Without a bassist, you need to sound the bass note yourself. For example, to play G/F# you might use the 2nd fret of the bottom E string, instead of the 3rd fret you'd normally use.
In addition to slim and NReilingh's answers:
Usually when you see a chord, the lowest note played is the root note of the chord. So when you normally see a chord marked as
Normally you play the chord going up. So in the
Musicians like playing around with something called "inversions" of a chord, where you change the ordering of the notes. Normally first inversion is where the second note (which in our example is the third degree of the scale) is played first:
In the same way, we have second inversion, which is where the fifth degree of the scale is played first:
So what the others were saying follows this logic. You should probably play an
Hope this helps!
Don't let the term "slash notation" bother you. It's simply a chord that has a bass note(the lowest note) that is not normally part of the chord. You find a lot of these chords in songs by The Beatles for instance.
While you are learning and so new at guitar, I suggest using songbooks that have chord diagrams each time you need to play the chord. In time, you won't need any help remembering the chords.