Cover Spring: The stainless steel spring is crafted from high grade stainless steel, and should never rust, unless stored next to your chlorine bucket, in which case surface oxidation crystals can appear. Cover Springs are 8" long, and should never need replacing, unless your cover experienced a heavy snow load, without the support of water within 18" from the safety cover.
Short Spring: At 5" in length, short springs are used on mesh or solid safety covers when the edge of the cover comes close to a wall or other obstruction. You can adjust the straps all the way to the end to position the long spring (8") up to the edge of the cover, but for those cases when you need just a few more inches to maintain cover tightness, the short spring is the answer.
Double Spring: Absolutely the strongest pool cover spring made. Ingenious design reduces bending and warping of the spring, and allows for more torque, without twisting the springs. If you need to really pull your cover tight, use the Double Spring. They also make it easier to put the cover on and off with the more open design.