C# 7sus4 (C#7sus4)
Also written as Db 7sus4 (Db and C# are the same key).
Notes in C# 7sus4
Inversions
Same notes as C# 7sus4, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
C# in the bass
1st inversion
F# in the bass
2nd inversion
G# in the bass
3rd inversion
B in the bass
Open voicings
A close-position chord like C# 7sus4 can sound cluttered in the middle of the keyboard. Jazz players open it up by dropping one note down an octave, spreading the same notes wider without changing the chord.
Drop 2
G# on the bottom
Drop 3
F# on the bottom
Press C#, F#, G# and B together on the keyboard and you're playing C# 7sus4.
A dominant seventh chord with the third swapped out for a perfect fourth, combining the open sound of a sus4 with the pull of a dominant seventh. Gospel and blues piano lean on it heavily, often as a stepping stone right before the full dominant seventh resolves.