http://www.durgan.org/URL/?QPBSG 30 May 2010 Self retracting clothesline.
Walking through a hardware store, I found what is the ideal string for making straight rows in a vegetable garden. The device is a self retracting clothes line. The string is about 3/16 inch in diameter, and quite strong. I did a sample run and found it to be most practicable.