Certain conventions are used to represent three-dimensional structures of organic molecules on paper. Using simple lines, solid (dark), and dashed wedges, a 3D image of a molecule can be perceived on two-dimensional paper.
A thick solid (or heavy) line or a solid wedge indicates a bond lying above the plane of the paper and projecting towards the observer.
A dashed wedge represents a bond lying below the plane of the paper and projecting away from the observer.
The bonds lying in the plane of the paper are shown using a normal line.
Other examples are n-butane with carbon atoms having tetrahedral geometry and 2-butene with trigonal geometry.
3D organic structure drawing rules- best practices
In the best bond-line depiction of a molecule, maintaining the geometry (or the shape) is very important. The best practice is when the pair of similar bonds (2 in-plane, 2 out-of-plane) is shown next to each other.
Tetrahedral Geometry
Trigonal Geometry
The double-bond carbon is planar, with a bond angle of 12o0. Also, the bond rotations around the double bond are restricted. The bond direction as wedges should always favour the geometry of the molecule.
Bond-line and wedge-dash drawing convention
The bond-line drawing conventions ignore carbon and hydrogen atom symbols from the organic structures. The wedge-dash diagrams many times follow similar drawing rules.
The H atom bonds are mostly omitted from the drawing and are assumed to be part of the structural diagrams.
In summary, the wedge-dash-line representation describes the position of various atoms in a molecule in space. Therefore, it is also called spatial formulas or three-dimensional structures.
The other ways of 3D representation on paper are Newman and Sawhorse projections.