>>        I got an "old" Digital Multimeter (DMM) that can measure ACV.
>>
>>        When I measure the ACV (with my old Multimeter) from the wall I
>>read 110V
>>or      220V (depending where I Use the Probe tips.)
>>
>>        So, I purchase a new DMM and in the box says: "TRUE RMS".
>>
>>        So I believe that ALL the DMM( the old ones and the new ones)
that can
>>measure ACV made it in RMS.     Is that true?

Many older or cheaper DMMs assume the AC they're measuring is perfectly
sinusoidal, and apply a fixed scaling factor to the internal reading to get
the "RMS" reading, which they then display. The "True RMS" DMMs supposedly
actually look at the waveform and don't assume it's sinusoidal. They
calculate an appropriate scaling factor and use that.

For example, reading a square wave will give a very different result on a
"True RMS" DMM than it will on a basic DMM.

Stephen.