The Phi Rectangle (1.618) vs. The 1.5 Rectangle
Many articles written on photography websites claim that the Rule of Thirds grid is derived from the golden section rectangle (Phi 1.618). This isn't the case. Also, because many photographers only use the Rule of Thirds and aren't aware of any other design systems, they will stretch the Phi rectangle to fit the shape of a 1.5 rectangle in hopes that it will give them a better composition. Unfortunately, by extending the Phi rectangle past its original physical dimensions, it changes the armature and the 90-degree intersecting diagonal lines. Simply put, once expanded, it's no longer a Phi rectangle. Below is an example showing the difference between the Phi rectangle and the 1.5 rectangle. Even though the dimensions of the two rectangles are close in size, notice the armature that each creates is drastically different.
In the photograph below, you can see how a photographer incorrectly overlays the Phi rectangle 1.618 (in red) on top of the 1.5 rectangle. This common error, which I see repeatedly on many of the popular social media-driven photography websites, is due to a lack of understanding and sufficient training in the basics of classical design techniques.
Photograph by Henri Cartier-Bresson designed using the armature of the 1.5 rectangle.