Riddle: My three brothers, my cousin and I are stood in a line. I am the shortest of all my brothers and I am at one end of the line. At the other end of the line is my cousin, who is the fattest out of all of us. My tallest brother is in the middle of the line. We cannot move away from where we stand and yet we are very flexible. We are very useful; people would struggle without us. What are we?