At times, many artists and photographers are unsure whether they should choose a horizontal or vertical frame for their compositions. If you shoot medium format negatives or you're an artist that primarily designs in a square, the question is irrelevant. However, if you are working with any dimensions wider than a square, meaning a rectangle, this dilemma needs to be addressed. Even though there is no simple answer, it's best always to consider your subject, the mood you want to portray, and the elements you want to lock into your chosen rectangle.
In the images below by Martine Franck, Henri Cartier-Bresson, and Erich Lessing, notice how they use their subject and the supporting elements within the frame to determine whether they shoot horizontally or vertically. For example, the top image of the girl laying on the ground is horizontal, along with the lines created by the row of cars. This repeated pattern of horizontal lines creates a visual rhythm.
The same can be said for the photograph in the second row. The figure is lying horizontally, echoing the top and bottom horizontals of the rectangle as well as the surface the subject is lying on. The subjects and mood of the scene successfully fit the chosen format.