Quick release skewers -They are significantly thinner than thru-axles, with a 5mm shaft and a small spring at each end. One side of the skewer always has a lever, while the other side has a cap nut. The skewer does not attach to the frame or fork -- ie. they do not have a threaded end.

For a front thru-axle, this is measured from the inside to the inside of your fork. For a rear thru-axle, this is measured from the inside to the inside of your frame at the drop-outs. The O.L.D. measurement is often listed for many thru-axles but isn't necessary if you know the overall length.

You can tell if your frame and fork are designed for quick release or a thru-axle by looking at how the wheels are attached. A thru-axle holds the wheel in a closed (or O-shaped) dropout, whilst a QR skewer holds a wheel in an open (or U-shaped) dropout.