This is an excerpt from Sports Rules Book 4th Edition PDF, The by Human Kinetics & Myles Schrag.
Hurdle events include the 60-meter (indoors only), 100-meter, 110-meter, and 400-meter races. Youth competitions can also include a 300-meter event.
Hurdle races are run entirely in lanes. A runner must clear the hurdle with both legs. The runner is disqualified if either leg dips lower than the horizontal plane of the hurdle. This means that the leg must go over the top of the hurdle and is not allowed to go around or beside it. Hurdlers are not disqualified, however, for unintentionally knocking over hurdles.
The top bar of the hurdle is made of wood or plastic. The hurdle is durable enough that it is not easily broken by a competitor hitting it. Heights of hurdles and distances between hurdles are shown in table 57.1.