All agree that the letter "Yod" requires a "Kos" (spike). Yet there are three different opinions what constitutes the "Kos." Some authorities learn that it refers to the right leg of the "Yod." The problem with this is that the leg is an integral part of the letter, whereas "Kos" implies that it is an additional adornment. Some understand that it refers to the spike rising from the top left of the letter. Others learn that it juts out from the bottom left. This opinion is brought by the Bet Yosef in the name of Rabbenu Tam. However, care must be exercised when putting a "Kos" on the bottom left so that it does not make the letter appear to be a "Het." Otherwise, it could become a Humra (stringency) which leads to a Kula (leniency). In fact, the Kaf HaHaim cites opinions in his Kol Ya’akob not to add the bottom "Kos." Practically, many Soferim are stringent and "hedge" to add all three versions to the Yod. Nevertheless, it is not Pasul (invalid) without the bottom "Kos," as long as the top "Kos" was done.
As one of the six letters of "BeDeK HaYaH," the "Yod" receives a single "Tag" (crown) on its top. The opinion of Arizal was to be careful to put the kos on the bottom left of the yud.
