Often, we must squeeze our fingers into some rather unaccommodating positions on the guitar.  We know we are supposed to strive to get our fingers "right behind the fret" as we play, but, like so many things in life, we have to bend the rules sometimes (it ain't a perfect world!).

For instance, I have always found the A major chord to be a naturally antagonizing chord shape. There is no way you are going to get all 3 fingers equally close to the fret. But, we do want to get them as close as we can, and to do that, we need to know that we must "overlap" the fingertips as they go down.

We don't want to allow them to line up side by side, as they tend to do, we must make them "overlap", especially 3 and 4. It looks life this:

A major chord down with finger "overlap".

By overlapping the fingers, we get the fingertips as close as possible to the fret in awkward situations like the A chord.

Another common, and important playing situation where we need this finger overlap is in doing the basic 5th and 6th string root bar chords. In fact, we want to be able to make this overlap happen even above the strings, as you approach the chord.

Here is what it looks like:

Finger overlap being used for 6th root Bar Chord.

Finger overlap being used for 5th root Bar Chord.

Actually floating above the strings before putting the fingers down, already in "overlap" shape.

Teaching a student to use "finger overlap" in bar chords.....

3 and 4 tend to spread apart in their approach to this Bar chord.
I am grabbing the students fingers and forcing them together, overlapping them. I am also bringing them as close as they can get to the fret.
Here is an exercise for building the muscles in the hand that bring the 4th finger into overlap position. Simply touch the pinky to the thumb. This strengthens the "Opponens Digiti Minimi" muscle, located in the palm. This muscle brings the pinky into overlap position.

One last word, and this falls in the "it ain't a perfect world" category: people vary in the shape and size of their fingertips. It is easier to do the finger overlap if you have more tapered fingertips. Some people have more "bulbous" shaped fingertips, and it will make it a bit harder to overlap, but you can still get a little more finger in a little less space by doing the overlap technique.