Audio Developments AD149 - Manual (Page 13)

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(Note: it is assumed that M=S, ie the microphones have the same sensitivity and that
there is the same amount of gain in each of the two signal paths.)
When M>S
M=omnidirectional: the resultant cardioids become more omnidirectional.
M=cardioid:
the rear lobes of the resultant hypercardioids are
reduced in size and the included angle decreases.
M=figure-of-eight: the included angle between the resultant crossed pair
is less than 90°.
When M<S
M=omnidirectional: the resultant cardioids become more hypercardioid.
M=cardioid: the rear lobes of the resultant hypercardioids are
increased in size and the included angle increases.
M=figure-of-eight: the included angle between the resultant crossed pair
is greater than 90°.
'All things being equal' (which, of course, they never are), once an M-S pair has been
installed, varying the input gain of the S-channel will not only alter the width of the
resultant L-R presentation, but also effect changes as outlined in the above table.
Whereas, if an X-Y pair is being used, changes would involve physical manipulation -
alterations to included angle, to position, or replacement with a different type of
microphone. (Refer to FIG 2)