If anything here is confusing, inspiring or absolutely incorrect your comments would be much appreciated.  This is a work in progress and your help improving the information is requested.  Thanks!

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As we survey various kinds of stereo mic arrays, it should be noted that precision and repeatability are important.  Very small changes in distance and angle can make a significant difference.  “Good enough for rock ‘n’ roll” may not be good enough for some of these arrays.  That is not to say there isn’t a tremendous amount of flexibility, just that you will probably want the ability to place mics by exact measurement.  And if you hope to achieve similar results the next time you record in those circumstances, being able to repeat the same setup, or slightly tweak it, is important.

I have obtained some tools over the years that help me position mics where and how I want them.  I found a tape measure that has both imperial (feet, inches) and metric (meters, centimeters) markings -- very handy.  A protractor can help angle mics precisely.  Reliable mic stands and shock mounts are also helpful to own.  The good people at Audio Engineering Associates make some fantastic mic positioning bars and systems that may be worth considering if you build a lot of stereo mic arrays. 

When we get to Near Coincident techniques we will discuss the Stereophonic Recording Angle, or SRA for short.  Being able to measure the width of your subject in degrees and compare that to the SRA can help you adjust your setup to the actual circumstances.  Michael Williams suggests ways that you can measure the angle, including building your own “Crocodile” angle viewfinder.  There are some nice little laser tools available at most hardware stores for measuring distance, surface area and volume, as well as angles.

For all of these arrays, it is assumed that the mics are placed relative to the centerline of the subject.  When using only two mics, their distance from the centerline should be the same.  For example, let’s say you are recording a choir and placing mics 6 feet apart.  The left mic should be 3 feet left of the choir’s center and the right mic should be 3 feet right of the choir’s center.

There are many different kinds of arrays.  The most basic ones are built using only two microphones, which can organize into four categories:

(1)    Spaced, also know as A/B

(2)    Coincident

(3)    Near Coincident

(4)    Baffled

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