Duplet - help me please

[feilin]

Is dotted note in duplet allowed? For example if the time signature is 4/4 and the notes are crotchet + (dotted quaver + semiquaver) + minim, and I should change the time signature into 12/8 without changing the rhythmic effect, is it right if I wrote : dotted crotchet + (dotted quaver + semiquaver) in duplet + dotted minim?

Thank you

[kalirren]

Yes, that is the most correct thing to do.  It leaves no ambiguity at all, even to a computer:

[mjedwards]

     It's a few months later; but I have thoughts on this which may be of interest, and I think more than one approach to this may be correct.
     I agree that the example given is correct, and can't be objected to, and my answer to the question "Is a dotted note in a duplet allowed?" is a definite yes.  But I personally would consider another approach, which on the whole I prefer.

     If I am writing something in 6/8 time, for example, the beats are dotted crotchets, and each is subdivided into three quavers.  If on occasion I wish to introduce a duplet, I do not normally write the two notes as undotted semiquavers with a "2" bracket over them.  Instead, I write the two notes as dotted quavers.  This gives exactly the same effect, but no duplet bracket is needed, because they are not tuplet-style variations of the normal rhythm, but perfectly regular dotted quavers.
     It can be argued that this subdivision is wrong in 6/8 time, and belongs to 12/16 instead, and so shouldn't be used.  But I actually think that, if a subdivision actually appears briefly in the rhythm that is irregular, it is completely appropriate to use the notation for the irregular subdivision - it merely mirrors the irregularity that is actually audible in the music.
     Similarly, if I wanted to divide the beat into four shorter notes instead of 6 notes (semiquavers), I would write the four notes as dotted semiquavers and no tuplet bracket, rather than four undotted semiquavers with a quadruplet bracket.  Similarly it could be considered wrong in 6/8 (I suppose it would be characteristic of 24/32 time, at a guess) - but, again, appropriate simply to reflect a real irregularity.
     That is the general approach I use with temporary duplet or quadruplet subdivisions in compound times.  If we use that approach here, it would mean that the example asked about here would need to be notated thus: dotted quaver tied to a dotted semiquaver for the first note of the duplet; and then another dotted semiquaver for the last note: three notes and a tie, instead of two in the duplet-bracket notation.  For some, that may be an argument for the latter notation, as given in the example quoted above.  Personally I would prefer the other notation just described, and certainly in simpler cases where tied notes ar enot involved.  In generally, I prefer to use regular notation that portrays rhythms according to the normal rules of notation (even if it's irregular in the sense of less usual); and I resort to tuplet notation if the regular rules don't cover the rhythmic subdivision I want.  But I generally avoid tuplet notation if an orthodox notation is available for the situation - which it is here, as described above.
     (All this would be far easier to show if I could post examples such as is done above - but unfortunately I have no means for doing so.)

     Yet another approach would be, as suggested (I think - I did find the original question a bit difficult to sort out), to change the time signature to something that fits the irregular rhythm.
     In general, I would do this only if the rhythm changed for longer - a few bars, at least - and, for those few bars, becomes the only subdivision.  Or I might do it for even just one bar *if* the irregular rhythm in the duplet were an important motif or thematic element that would recur from time to time - certainly not for a little bit of decoration or figuration.  But in general, my dotted-note, non-tuplet approach, as previously detailed, is the default I use.

Regards, Michael.