Wikipedia
The midwoofer-tweeter-midwoofer loudspeaker configuration (called MTM, for short) was created by Joseph D'Appolito as a way of correcting the inherent lobe tilting of a typical mid-tweeter (MT) configuration, at the crossover frequency, unless time-aligned. In the MTM arrangement the loudspeaker uses three drivers: Two mid-range (or mid-woofer/woofer) for the low frequencies and a tweeter for the higher frequencies, with the tweeter being placed between the mid-range drivers (as shown in the second image below). D'Appolito initially configured his design using a 3rd order (18 dB/oct or 60 dB/dec) crossover, D'Appolito has since amended this original recommendation in favor of 4th order topology. However, this does not impart any significant effect on the MTM design's unique characteristics.
There are speakers where the tweeter appears below the larger driver and though electrically it is identical to the MT configuration, it is customarily denoted as "TM". The only other difference is the lobe tilt which is exactly the opposite of the MT configuration.
The direction in which the lobe tilts (i.e., the vertical orientation or angle of the lobe) is a function of the difference or offset between the acoustic centers of the two drivers.