In the nomenclature of fused arenes you need to add a 'fusion locant'. The first face should correspond to positions 1 and 2 and is lettered ‘a’, continuing alphabetically clockwise around the exterior of the parent structure. In this case, the parent is naphthalene and using numbering of naphthalene the benzo is fused on position d so the name would be 1H-benzo[d]naphthalene
But IUPAC also has some rules about the orientation of such structures: (a) maximum number of rings in a horizontal row (b) the maximum number of rings in the upper right quadrant. (See the drawing on the right.) Subsequent the numbering of peripheral atoms in the preferred orientation starts from the uppermost ring. (If there is more than one uppermost ring, the ring furthest to the right is chosen.) Using that numbering scheme the face of fusion should be i.
![1*H*-benzo[*d*]naphthalene or 1*H*-benzo[*i*]naphthalene](https://i.stack.imgur.com/tew3D.jpg)
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