Twin Beds

I am a faternal twin and when I was young, my sister and I shared a double bed. Since it was made for two people...and twins are two people I always thought it was called a twin bed. That is until one day when I was getting sheets from the closet and found that the twin sheets did not fit! I could not understand how a single bed could be called a twin, when it was only for one person. My mother was eventually able to explain it to me.