I suppose this may be something that is obvious to most people, but I didn't think of it in advance, so I thought I would start a thread. As I mentioned in another thread, I've been recording very soft acoustic guitar in M/S with Beyer 160/130 ribbons. I decided to try recording a bit closer - moved in to about 12 inches. The sound was good, but when I decoded the M/S the sound was panned very hard to the left. After scratching my head for a few minutes I started slapping it. Of course, when two coincident mics are very close to the instrument they are going to pick up essentially the same signal no matter where they are pointed. With XY or Blumlein this means not much of stereo field, but nothing anomalous. With M/S the M+S side is phase reinforcing, the M+S canceling. About 9dB difference in level. No problem - I'll just use the 160 in mono, which sounds very nice. But I had never heard about this before (and hadn't thought it through in advance) so I thought I'd throw it out there.