A First Inversion Triad has the third of the chord in the bass rather than the root. It is notated with a letter e.g. Ib (Ia being root position) or with figured bass style numbers indicating the intervals above the bass that complete the First Inversion Chord e.g. I6 - this indicates that the third of the chord is in the bass, the root is a sixth above the base and (it is implied that) the remaining note (the fifth) is a third above the bass. First inversion chords can also be called Chord of the Sixth or 6-3 chords too. When harmonizing for four voices SATB one of the voices must be doubled. In the exercises that follow, you should adopt the following rules for your use of First Inversion Chords:
Double the root or the fifth (doubling of the third is generally to be avoided but we will loosen this rule later)
Keep the Soprano moving where possible and keep the common tone in an inner voice - this helps shape your melody line
If you repeat the Soprano note then change the inversion rather than changeing to a new chord
Using some or all chords in Open Position will give a better sounding solution
When going from IV to V or vice versa, if either is an inversion then contrary motion is not required between the bass and the other voices