Just as we can use non harmonic notes in a chord, we can do a similar thing among chords.
Besides a neighbor tone there can be a neighbor chord
Besides a passing tone, there can be a passing chord
Besides a suspension, there can be a suspension chord
We discuss three forms connected with the main triads of I, IV and V
I64-V-I (the suspension 64 chord)
I-IV64-I (the neighbor 64 chord)
I-V64-I6 or I6-V64-I the passing 64 chord
I-V43-I6-V43-I the passing 43 chord