The Halacha is that seven letters in a Sefer Torah must have Tagin (crowns) adorning their top. Those letters are known by the acronym of "ShATNeZ GeS"-the Shin, Ayin, Tet, Nun, Zayin, Gimmel and Sadi. The shape of four of those letters, the Shin, Ayin, Tet and Sadi, has the form of the letter Yud on their left side. On those letter the Tagin are placed on the left side.
The shape of the Tagin is three little lines, drawn in the form of "Ziyunin"-meaning in the shape of miniature letter Zayin. The Mishna Berura writes that if one drew straight lines, instead of a Zayin, it is still considered a kosher crown.
Sometimes, there is the letter Dalet above a letter requiring Tagin. In such a case, care must be taken not to draw the crown too long, to prevent it from entering the space of the Dalet on top and converting it into the letter Heh. Similarly, if the Tag enters the space of a Kaf Sofit, it could turn it into a Koof.
