Kirchhoff's Voltage Law

What is loop?
A loop is any path that goes from node to node and returns to the starting node, passing only once through each node.

Kirchhoff's Voltage Law:

The sum of the voltage drops around any loop equals the sum of the voltage rises.

or

The algebraic sum of all voltage drops around any loop equals zero.

Mathematically,

v1 + v2 + v3 + v4 = 0
or

Parallel Connection:
Two or more elements are in parallel when their terminals are connected to the same pair of nodes.

Also that elements in parallel have the same voltage across each one of them.