For those who don't know or if I got the name wrong the string test is when you measure the front leg of a young horse and double the length to determine height the horse will be when fully grown. It doesn't work on pre-mature foals until or twins until they are a year old or so. But for other baby horses I find it works almost from birth. Measure the leg with a string and double it to tell you how tall your horse will be at the withers. I have always found it to be accurate.