Music notation software have solved most transposition problems. However the choice of a piece tonality according to a transposing instruments panel remains an issue; so does the choice of a transposing instrument according to the tonality of the piece.
Transposition is a simple transfer of the fifths cycle.Let’s imagine a sliding rule:The fixed (yellow) part reads tonalities of the piece while the sliding (blue) part reads the instrument tonalities.
So willing to write an obsolete Db piccolo part for an Bb (2b) melody
I place the instrument C facing Db(piccolo tonality) on the fixed part ;to Bb (piece tonality) tallies (A) instrument(blue) tonality.
This needless method for a single instrument becomes a precious tool when several transposing instruments are involved.
First of all it should be reminded that the writing confines tonalities to 7 accidentals(From Cb(7b) to C#(7#)) in order to keep a relative legibility; Beyond those limits we use enharmonic writing
« Forbidden » zones are in red but enharmonic choice is also advisable if an easier alternative tonality is available.
For example , let’s take an Ab Maj (4b) piece
For an A clarinet the chart reads 7b key signature
The best choice would be to switch to a Bb clarinet (2b)
But ,should we use an A clarinet for any reason ,we ‘d choose the B Maj (5#) enharmonic writing.
Below is a correspondence chart
Neglecting the classical double flats/sharps notation we suppose an unlimited number of flats/sharps instead.
The result is always 12.
Double sharps equivalence
Notice the reverse direction of increasing flats and sharps