The joint between separate faces of a solid is called an __________.

In solid geometry, a three-dimensional figure is built from three basic elements: a face (a flat surface), an edge (the line segment along which two faces meet), and a vertex (the point where edges meet).

Here, the two separate faces of the solid come together along a shared straight line segment. By the definition above, that shared boundary line is exactly what is called an edge.

• Face names one of the flat surfaces itself, not the boundary shared between two such surfaces.

• Vertex names the corner point where several edges meet, not the boundary shared between two surfaces.

• None of these does not hold, since one of the listed terms does correctly name this boundary.

Therefore, the joint between separate faces of a solid is called an edge.