σ-Bonds :- One in which electron density is arranged symmetrically around the axis connecting the two bonding C-C/C-H atoms.
Since electron density lies along the axis the extent of orbital overlap is not significantly affected if the atoms rotate about the inter-nuclear axis. Thus, “free rotation” is the norm. This free rotation is the cause behind the different types of isomeric structures seen in different members of a homologous series.
The 3D character of sp3-hybridized atoms in alkenes by using single lines to represent σ-bonds.
Solid wedges to indicate those coming towards the observer
Hatched lines to indicate those going away from the observer (figure below)
This method of representation is sawhorse representation