Final answer:
A Triangular shape haircut typically begins at the nape of the neck, creating a strong baseline for the haircut to build upon with longer lengths towards the front.
Step-by-step explanation:
When beginning a Triangular shape haircut, the typical starting point is b) At the nape of the neck. This is because the nape is the lowest point of the head and starting here allows for the creation of a strong baseline from which the rest of the haircut can be built upon. The triangular shape is achieved by cutting the hair shorter at the nape and gradually getting longer towards the front, usually culminating in longer lengths around the face or the chin, depending on the desired final look.