Yet, not all intervals of the same numerical classification are of the same size. That is why we need to specify the quality by finding the exact
number of **whole** and **half steps** in the interval.

From any piano key to the next we have a half step:

Some seconds have 2 half steps or a whole step and some seconds have 1 half step. In the following example the second from C to D has a whole step while the one from B to C has only a half step:

The same thing happens with other intervals. That is why we need to specify the quality of the interval by finding the exact number of whole and half steps.

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José Rodríguez Alvira.