A double bond is made up of one sigma bond and one pi bond.

Sigma Bond:
- A sigma (σ) bond is formed by the end-to-end overlapping of atomic orbitals.
- It is a strong bond that allows the rotation of atoms around the bond axis.
- It is a head-on overlap of atomic orbitals, resulting in the formation of a cylindrical electron density region between the two nuclei.

Pi Bond:
- A pi (π) bond is formed by the sideways overlapping of atomic orbitals.
- It is a weaker bond compared to the sigma bond and restricts the rotation of atoms around the bond axis.
- It is formed by the overlap of two parallel p orbitals or one p orbital and one d orbital.
- The electron density is above and below the bond axis, creating a cloud of electron density known as the pi bond.

Formation of a Double Bond:
- A double bond is formed when two atoms share two pairs of electrons.
- The first pair of electrons is shared by the end-to-end overlap of atomic orbitals, forming a sigma bond.
- The second pair of electrons is shared by the sideways overlap of atomic orbitals, forming a pi bond.
- The combination of a sigma bond and a pi bond forms a double bond.

Example:
- Let's take the example of ethene (C2H4).
- Each carbon atom in ethene forms three sigma bonds, two with hydrogen atoms and one with the other carbon atom.
- In addition to the three sigma bonds, each carbon atom also forms a pi bond with the other carbon atom.
- The sigma bond is formed by the end-to-end overlap of sp2 hybrid orbitals, while the pi bond is formed by the sideways overlap of p orbitals.
- The double bond between the carbon atoms in ethene consists of one sigma bond and one pi bond.

In conclusion, a double bond is made up of one sigma bond and one pi bond. The sigma bond is formed by the end-to-end overlap of atomic orbitals, while the pi bond is formed by the sideways overlap of atomic orbitals.