The Letter Yud is the smallest letter of the alphabet, yet it can be the trickiest to write properly. The Yud should have a size of 1X2 kulmusim. The Yud should have a small Tag on the top left corner. This Tag should not be made too tall, because it can cause the letter to be invalid. The top right corner should be clearly rounded. The leg should not be too long, because otherwise the Yud can turn into a Vov. There should be a small kotz sticking out of the bottom left corner of the head of the Yud. According to many Poskim, a Yud without this kotz is invalid. A kotz is a small stick or thorn, and in this case it's used to represent a small point sticking out of the letter. Altough it resembles two yuds written together it is indeed one single letter. The line under the yuds is called Pesakh and gives the letter mthe sound of an "i" as in light.