Hello:

I only play by ear, I have been doing this for the last 40 years (I began playing pro VERY young, 12 yo to be precise),in a Restaurant in my home town long time ago.
I now use 2 arranger keyboards, and only styles, no MP3s at all, and as far goes SMFs, I don't use them, unless I made them for a specific situation.
I have learn to read chord charts only, Cmaj7, Fdim, Esus4, etc..Software like Cakewalk, Power Tracks, BiB helped me to learn.

Some of my friends and colleagues, have told me that I have perfect pitch, I guess I can hear in what key is been play X song, and I learn those songs with all the chords and most of them in the original key too, sometimes when I was the accompanist for a singer that the song was not in a suitable key, I transposed manually, that was when the then Organs did not have a transpose function.
When I have became very familiar with the songs I add more chord to the songs; and I guess that they are correct, since I have perform along side with a Jazz pianist in the area (as a duet and with his band too),and he ask me what chords I have added to the wherever song we are playing, he likes them, and he includes those chords in his arrangements.
I play in many keys, and of course, "C" too.

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mdorantes