Can someone help me with compound time?

[boxey194]

Okay, so I know what compound time is, like 6/8 9/8 or 12/8, but I'm not sure what to set my metronome to. Let's say a pieces is allegretto and is in compound time, and let's say allegretto is around 110, would that be per quarter note or per eigth note. I always thought it was per eighth note, and therefore compound time is "slower", so eighth notes would be slower in 6/8 than they would be in 3/4. Is this true?

[chopinlover01]

For that, I generally try and figure out what the pulse of the music. I once had to play a new age style nocturne (it sucked, boring as hell!) that was marked moderato. It was marked 4/4 time, but the pulse was clearly in eight notes, so it should've been marked as quaver=100 or whatever the hell the MM tempo was.

[quantum]

Taking a step back for a moment, let's clarify:

Simple time: main beat divisible by 2
Compound time: main beat divisible by 3

An example of simple duple time (two beats in a bar) is 2/4, where the main beat is a quarter.
An example of compound duple time (two beats in a bar) is 6/8, where the main beat is a dotted quarter.

Metronome markings come in two parts: a note value = a numerical value in bpm.  A numerical value by itself would have no context in which to place it. 

Taking your example, an allegretto in compound duple (6/8) would most likely be felt as two beats in a bar, with a corresponding metronome marking for the main beat which is a dotted quarter.  So a dotted quarter = 110.  If it were allegretto in simple duple (2/4), the quarter note would get the beat and the corresponding marking would be: quarter = 110. 

Slower music may specify subdivisions as a metronome marking to enhance precision, due to the faster clicks of the metronome.  Example: a grave 6/8 may specify the eighth note with metronome markings. 

Faster music may specify an entire bar of music for one metronome click.  Example: a presto in 3/4 using a dotted half for metronome indications.