Believe it or not, you can turn your camera on its side and it will still work! So many beginners will not even think
about the vertical format and end up with a lot of empty space on the sides of the subject that could have been
eliminated by simply going with a vertical format.
Some subjects are obviously suited to a vertical format - anything that is taller than it is wide. But, having said that, a tall subject could also
work in a horizontal format. Take a waterfall, for instance. If you want to show the whole waterfall, fairly close in, use
a vertical format. But maybe you want to show the setting around it as well. So you back up or use a wider angle lens
and include some of the rest of the scene in the image. (Remember what I said about rules being simply guidelines
and not to be strictly adhered to?).
Let's look at these two pictures. Same subject, same vantage point. Both formats work. It depends on what you
want to say.