2. Sus9 chord written as slash chord
F/G for Gsus9 =G7sus4(9)
First the F major triad: we try to find the root of the F major triad on the D, G and B string and end up with the F on the third (III.) fret on D-string. We play an F major triad in the base form (root, third, fifth) as shown in the chord diagram.
Then we are searching for the bass tone on the lowest two strings and we find the G on the third (III.) fret on the E string.
C/D for Dsus9 =D7sus4(9)
C major chord: we find the C on the fifth (V.) fret on the G string and play a C major triad in the base form, this time over the highest 3 strings. That just sounds better in the case of this chord.
Bass tone: we play the D on the fifth (V.) fret on the A string.
Mostly replacement for the major chord on the 5th scale degree of a key: e.g. F/G instead of G in a song in the key of C major.