By using the keyboard to count the number of half steps between notes we can see that intervals with the same numerical classification can contain a different number of half steps. For example, the second between C and D has one whole step while the second between E and F has only one half step:
The same thing happens with other intervals. For example, the third D-F has 1 1/2 whole steps or 3 half steps, while C-E has 2 whole steps or 4 half steps:
This is the reason why we need to specify the quality of an interval. Please refer to each interval for more information.