These are three of the pens roaming around my desk that I'll grab when I need to sketch something. I don't love any of them but I got them all for free.
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They're all ballpoints that deploy the tip when a portion of the pen is rotated. That's a nifty and timeless feature, but London-based design firm Beyond Object has done the classic mechanism one better, adding a visual and tactile trick that gives you the satisfaction of correcting the pen's interrupted form in order to deploy the tip: