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 two half steps or a whole step and some seconds have one 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 interval's quality by finding the exact number of whole and half steps.

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