I don't know if it makes any difference, but all my ballasts were originally vertical bulb units. So you don't think it will cause any harm if I switch it over to horizontal and paying no attention to bulb position? I was curious as to why the sockets for the horizontal ones specified bulb position. They are only 400w and don't throw off near as much heat as all my 1000w do, so I thought it would be okay to convert. I just read justa's second part and I think I might try to contact a bulb or ballast manufacturer and see what they say. All of the ballasts I have were made only about 10 miles from here so I might give them a shout on Monday morning. I have seen many MH parking lot and security lights mounted sideways but was told you shouldn't. Thanks again.