IT'S ALL SMOKE AND MIRRORS.
When learning to play the trumpet many teachers start with the note C. This is an open note and can be played without valves. The next highest note with the same fingering (open) is a whole fifth up. This is the next "natural lip position" on this instrument. All the notes in-between these two notes (1-5) are created using the valves. On the staff the trumpet player is taught to read these notes as "C" and "G." Now we know that the sounds created by the trumpet are really the notes "Bb" and "F" as played on a piano. The trumpet is a transposable instrument. When playing the trumpet the player must know the music they are reading is written a whole step higher then the instrument is sounding. I.E. If the music script is in the key of "G" then the horn is sounding the "F" scale on a piano, guitar, flute, recorder, all the strings, oboe, banjo and accordion Etc. When you lookup the fingerings for an "A" scale in a trumpet book you must remember that you are sounding the key of "G." In the fingering charts that follow we are only using concert pitch and what you see is how it sounds.
Open "Bb" is to high for most beginners to get any higher then "F" and it takes a long time to get a bandwidth of one octave on the trumpet. I get my students to start on the low "G" (A to trumpeters) and in a fairly short time there playing a full octave. Learning to play in the key of "G" works great for songs that go from 1 - 1. By adding a "C#" you can move the one (DO) half way up the scale and play in the key of "D" for songs that go from 5 - 5 and remain in the same playable bandwidth. Learning to play in just two scales easily and well, will allow you to play most songs. Reading the music script by number in any key you can instantly translate that song into your instruments bandwidth in either "G" or "D."