I use both thumbs with the tips of my nails. When I am really "flying" in an email, I find myself tapping in a rythym! I didn't realize this until just recently when I noticed a nice little percussion beat. I actually think it keeps my error count down.
It depends on how I'm holding the Curve at that time. If I'm holding it with one hand (usually with just the right hand althought I can also do left) then I'd use just the right thumb to type. I've gotten pretty fast at it too. lol
If I'm holding it with 2 hands then it's usually both tips of my thumbs.
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