When the cool winds blow and the starts are early, cycling knee & leg warmers are cycling essentials. Designed primarily to keep your lower thighs & calves from getting cold, they are high on the list for 'most used item' in a serious road cyclist's wardrobe.
Cycling sometimes requires pretty impressive powers of weather detection, and it is with these powers that you will decide if the day's ride requires knee or leg warmers. If you're only just acquiring these skills then we can make it easy for you. If it's cold enough to make your lower legs tingle and the hairs stand on end, then go back inside and put on some warmers - easy! Alternatively, ride without warmers but keep them balled up in a rear jersey pocket, in which case you are covered for both eventualities.
Choosing a knee or leg warmer for cycling is quite easy, and like many things in life, you just have to ask yourself what you are looking for. Some riders prefer a stretchier material, and aren't overly concerned with weather protection. On the other hand, there are riders who like to have a degree of wind and water resistance to their warmers. Finally there are riders who prefer natural fibres like merino wool, but others swear by synthetic offerings.