about an inch from the stem on the underside. I talked to my local bike mechanic and he suggested it here as that is were this set of bullhorns is double thick. He also said that the underside would be better as compression from pushing down was less likely to cause cracks than stretching forces. So far I havent had any problems. They are really small holes just big enough for the cable housing. I know a lot of people have differing opinions though on if it is wise to drill especially so close to the stem. I dont predict having any problems but Im checking them regularly just to make sure.