Is there a difference between horizontal bulbs and vertical bulbs?

In other words: can you hang a vertical lamp horizontally, I didn't think you could.
or is it just labeled vertical or horizontal due to the type reflector/hood it is mounted in?...thanks to all who reply :)

Indeed, some lamps are designed for strict situatuions.

There are indeed horizontal and vertical ONLY lamps.

At least that is how they are marketed.

Now ask me what specifically makes them such, I have not the slightest idea and I have never 'altered' the reccomended mounting layout....

Curiosity killed the cat....