The Rule of Thirds is often considered the most important rule in photographic composition. It states that important objects should be placed at the four points which mark a third of the horizontal and vertical diameter of a picture.
I do not like the term “composition” and prefer to talk about the structure of a picture. And I often found the Rule of Thirds not very helpful for pictures in landscape format. But in my opinion this rule is commonly misleading for pictures in portrait format.
Pictures in Portrait format are very narrow. The most logical place for the center of interest is usually in the middle and not on a third. These narrow pictures tend to guide The Eye vertically and are not very good for horizontal storytelling.
Of course there are pictures with several interesting objects or subjects which need to be balanced and could make it better to deviate from the midline. But the edge of the picture might be as good as choosing a third.
The vertical axis is more important in such a narrow picture. The artist should guide the viewer vertically through the picture. Portrait format is called so because it is good for portraits. I don’t have an example at the moment but the face should be in the middle in a headshot. Deviating and placing it above or below needs a reason.
I prefer to place human figures with their feet near the lower edge of portrait format pictures. It appears to me as the most natural place. In the picture below the lady is higher as the colourful reflections lead vertically towards her.
And placing the centre of interest on the lower edge dramatically reinforces the feeling of height in a picture.