* The subject and composition: A tightly framed head-and-shoulders portrait often works well in portrait orientation. However, a full-length portrait or one emphasizing a sweeping landscape behind the subject might look better in landscape (horizontal) orientation. The composition itself dictates the best fit. A wide landscape behind a person, for example, might be cropped too much in portrait.
* The intended use: A portrait for a driver's license needs to fit specific dimensions, often portrait. A portrait for a website banner might require landscape orientation to fit the space effectively.
* The artistic intent: Some photographers deliberately choose landscape orientation to create a specific mood or emphasize a particular element within the image. It can create a different feeling than a vertical shot.
In short, while portrait orientation is common and often works well for portraits, it's not a hard and fast rule. The best orientation is the one that best serves the image's composition, intended use, and the photographer's artistic vision.