Sewn 3-dimensionally to fit the shape of your finger.
Two layers of leather for increased safety.
These are the only thimbles I have got on with, they come to the shape of your finger like a leather shoe to your foot.
For sizing you need to measure the first joint on the finger the thimble is going on. Wrap a small strip of paper around your finger joint mark it and measure.
Small is 14.5mm
Medium is 16mm
Large is 17.5mm