Why hexagonal closed packed structure is not a Bravais lattice?

Why is the hexagonal closed packed structure not a Bravais lattice?

How can one readily say that a particular lattice is Bravais lattice or not?

• A Bravais lattice is a well defined concept in solid state physics, covered in many textbooks. A HCP structure is a simple hexagonal Bravais lattice with a two-atom basis. Aug 28, 2018 at 18:33

2. Position vector of any discrete point in the lattice can be written in the form $$\mathbf{R}=n_1 \mathbf{a_1}+ n_2 \mathbf{a_2}$$ where $$\mathbf{a_1} ,\mathbf{a_2}$$ are primitive, linearly independent vectors which spans the whole lattice.