It is important that you fully understand what has gone before because this next bit is where it starts to get a little more complicated. But is still easy if you have grasped what has gone on before.
If we continue with our example scale of C, we know that the second interval note will be D. However, thinking back to our scale pattern of intervals we know that the interval between C and D is a full tone which means there will be another note that we have not yet given a name to that lies between C and D. Remember that there were a total of 12 notes in the octave that were each a semitone apart, from which we chose just 7 to make our scale.
So a semitone up from C will be C# (sharp) and a further semitone up will take us to D. So taking that example a little further we can see that all intervals in our scale pattern that are a tone apart, will have a further note that as yet we have not named.
So, starting again at C but moving up the scale, the notes will be C, C#, D, D#, E, F (because between E and F or the third and fourth intervals is only a semitone) F#,G,G#,A,A#,B,C in total twelve 'different' notes for the complete scale of C major.
Let me say at this stage that there are in music, many different types of modes or scales that do not follow this TTSTTTS pattern, but well over 90% of our barbershop music does not use any of these these different modes apart from one, so at this stage we can ignore them. OK musicians?
So we have now looked at the various steps in the scale and these steps will remain identical for every key the scale is in.
It matters not if we are singing in the key of Ab (four flats) or B ( five sharps). The steps of TTSTTTS will remain identical for ALL keys. And once you understand this you will have made great strides towards sight singing with virtually no effort damage to your brain.