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do you like using the altenative fingering for Bb?
I've tried that alternative and it sounds beautiful! I've tried to use it in the "flat" scales/arps that I'm practising but it's very difficult. Is it possible to use it in scales with practise, or is it best to keep it for pieces which are fairly slow?
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The alternative Bb is great in pieces that are fairly slow, and you're not going to/from a note that requires a lot of fingers.
Scale wise, it's awkward to use in most, as inevitably you've got a note such as B or C to play afterwards. Putting some right hand fingers down does help a bit with the regular fingering for Bb, and you can use the alternative for one octave of Bb major, a set scale for grade 3.-------------------------------------------------------------------
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do you prefer using left hand or right hand C, or are you 'little finger ambidextrous'?
I always use the right hand C. Now that I've written that, I can see that I probably ought to practice using the left hand C! At the moment I only use the left hand C when I have to, eg if I need my RH for an Eb.
I used to find the right hand clarion C# really awkward, but now that the thumbrest is higher up, I find it quite easy now.
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It is well worth practising to be 'little finger ambidextrous', if you can. Very useful when you're playing at speed. 
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The 'long' Eb/Bb fingering you mention is a fingering I tend to only use when it's easier to do that. I do use the right hand banana key quite a lot (for 'fork' chalumeau B/clarion F#).
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For some reason I quite like using the "long" Eb/Bb key. I wonder why it is that you actively *enjoy* using some keys

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To take the 'D fingering plus 4th trill key' Eb as an example, Eb will often have D preceding/following it, and it's just so much easier in that case to get to the 4th trill key fingering than 'long' Eb (1/1) I find, especially at speed. 
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Edit : the quotes don't seem to have worked properly. I've put dotted lines between everything, and made my new replies bold to make it a little easier to read (I hope)....