F major and D major

In the preceding example you can see how all notes are one whole step away from each other except from the 3rd to the 4th note, and from the 7th to the 8th note. 
In case of the C major scale the natural half steps from B-C and E-F happen to fall into the right places. This is not always the case though. 
If for example you would start a major scale on a G everything would work out fine, until you get to the 7th and 8th step. In order to make the distance between these two notes a half step, as the formula requires, we would have to add a sharp in front of the F.





Example:

This, by the way, is how key signatures came about.
The key signature for G major would be one sharp:

Two more examples:

F Major:

So the key signature of F major would be:

D major:

Key signature of F major

With the knowledge you have gained in this section, you are able to figure to the key signature for any scale.

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