Single layer straps cut from thicker leathers, typically softer and more pliable than a lined strap.
One piece of leather carries the whole strap, with no lining to stiffen it, so it softens quickly and takes the shape of your wrist.
An unlined strap is cut as a single layer from thicker leather, with no lining behind it. Finished it runs 1.6 to 2.0mm, and there's nothing in it to stiffen the bend. That's why we cut it in leathers with enough body to carry a strap on their own.
One layer, no backing, so it starts moving almost immediately. It softens over the first few weeks and takes the shape of your wrist instead of asking your wrist to meet it. A lined strap is the pick if you'd rather have structure that lasts and a line that stays crisp.
Shell cordovan is the leather we cut this way. It has the body to carry a strap on its own, and it is far more pliable than that mirror surface suggests, which is what makes it the one worth cutting unlined. Every other leather we cut comes lined.