Picking leaf lettuce???

I am new to planting leaf lettuce. When do you start harvesting leaf lettuce? How can you tell it is ready? Do you start by picking the bottom leaves or the new tender leaves at the top?
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You take leaf lettuce when it's big enough and has not begun to toughen. Leaf lettuce is pretty tender stuff. No need to be selective when you take it. Cut it off at the base with a knife. I like the Black Seeded Simpson myself.
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I think I got Butternut, is it any different?
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When you cut it off does it regenerate itself for successive cuttings? I also have Buttcrunch.

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is not a true loose leaf.
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