A circuit is usually obtained from different elements connected one to another, so that there is one or more routes to close the electrical current.

We will consider the circuit from figure 1, formed by a power supply and 3 elements.

Figure 1 – Circuit with a power supply and 3 elements

In order to realize this circuit, there have been made some connections between the terminals of the elements; these connections are called “junctions”. We therefore have:

• one of the terminals of the power supply has been connected to one of the terminals of the 1^st element (junction A)
• the other terminal of the 1^st element has been connected to one of the terminals of the 2^nd element and to one of the terminals of the 3^rd element (junction B)
• finally, the other terminals of the 2^nd and 3^rd elements, are connected to the free terminal of the power supply, thus closing the circuit (junction C).

As a diagram, a circuit can be represented as we can see in the following figure:

Figur 2 – The circuit from figure 1, redrawn

From the representation's point of view, the junction B has been "divided" in the junctions B1 and B2; as the potential of a point is unique, the potential difference between B1 and B2 is zero; it is as if a perfect conductor would connect the two points. Therefore, the points B1 and B2 are considered a single junction. An identical reasoning can be applied to the junction C. The circuit has therefore 3 junctions: A, B and C.

Passing through the junctions, we can identify circuits in which the current can circulate; these are called “loops”.

The considered circuit has 3 loops, as we can see from the following figure:

Figure 3 – Identification of the loops from Figure 1

• a loop, represented with red, which closes itself by using element 1, then element 3 and finally the power supply;
• a loop, represented with blue, which passes across element 1, then element 2 and closes through the power supply;
• finally, a loop represented with green, which goes around element 3 and closes through element 2.

Any of these loops can be traversed by an electric current.