The A form bar chord (strum only strings 5-1):
The A form bar chord is a little more tricky, and there are perhaps a few ways to perform it. Using the same fingers, 2-3-4, chord the normal A chord. Then, slide all fingers up 1 fret (closer to the body). Lay the index finger across all 6 strings on the first fret. This is the A form bar chord playing a B flat.
Another way to create the A form chord is to use only two fingers, the index (#1) and the ring finger (#3). In this technique, bar all 6 strings in the same fret with the index finger. Next, skip 1 fret and bar only strings 2-3-4 by bending the ring finger to chord all 3 of these strings at once.
The Am form bar chord (strum only strings 5-1):
Using the same directions as in the previous paragraph for the A form bar chord, create the normal Am chord using the fingers 2-3-4. Notice that if you refer to previous lessons on chords, you will see we are using alternate forms of chords E, Em, A and Am. Now, slide the fingers up 1 fret (closer to the body of the acoustic guitar) and bar all 6 strings (or just strings 1-5) with the index finger again.