The FCI JRT head shape cannot be correctly described as triangular
Corgi Head
The Pembroke Corgi has a true equilateral head shape with the muzzle tapering to a point. This head is described as "Fox like". Note: the triangle shaped head places the eyes to the side of the head rather than the front, which is not suitable for an earthdog. The Corgi was bred to be a herding dog.
This FCI registered Jack Russell Terrier 's head doesn't fit in the triangle.
This clearly shows that this FCI/ANKC registered Jack Russell Terrier's head does not fit into a triangle.
Incorrect Triangle shaped head with a muzzle that tapers to a point. Nothing about the correct head shape should should resemble a triangle shape.