Temperature outside is 59° and below:  Your horse needs a sheet.

This would be the equivalent of you wearing a long sleeve shirt.  If your horse is in a stall, choose a cotton sheet.  If your horse is turned out, opt for a turn-out sheet for its durability and in case it rains.

Temperature outside is 49° and below: Your horse needs a blanket.

This would be the equivalent of your wearing a winter coat.  Turn-out blankets are the best option because they are waterproof in case of inclement weather and for their durability.

Temperature outside is 39° and below: Your horse needs a blanket and a sheet.

Layer your blanket and your sheet on your horse together to keep your horse toasty.

Temperature outside is 32° and below: Your horse needs a blanket, sheet, and a liner.

Layer your blanket, sheet and a liner such as a cooler, a second sheet, or a blanket liner for those freezing temperatures.