Are MT1 and MT2 of a triac interchangeable ?

I see circuits with either MT1 or (more often) MT2 to phase. I
don't use triacs a lot, but do recall making a heater control (an
immersion element) that didn't work with MT1 to phase but did
with MT2 to phase. Gate control was DC from a CMOS gate,
a 4017 I think. Are the positions of MT1 and MT2 less important
if gate control is AC, through a diac for example ?

When I've used low-power optotriacs like MOC302x to drive
triacs, I've never had a diac. Gate current is AC via a resistor
from phase through the opto's triac

In this document

Thyristors & Triacs - Ten Golden Rules for Success In Your Application

www.nxp.com/documents/application.../AN_GOLDEN_RULES.pdf

MT2 is to phase, as it is here

http://en.wikipedia.org/wiki/TRIAC

with note that filters should be referrred to MT1, likening it to the
'cathode' of an SCR, implying that MT1 is to neutral

In the previous version of his dimmer

http://www.circuitstoday.com/simple-lamp-dimmer-fan-regulator

the author has MT2 to phase, but in the revision of my original
post MT1 and MT2 are not identified

I'm asking for general knowledge and so that I can make a
small controller PCB

TIA

Joe
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