Different fingerings for each size of instrument
We mentioned above that on a C melody saxophone, the C scale sounds the same as the C scale on a piano. So you start the scale with three fingers of your left hand and four fingers of your right.
However on a tenor, which has a slightly longer tube, the note that comes out with that fingering is a Bb, and on the alto it is an Eb.
This means if you learn alto fingerings, but then want switch to tenor, you would need to learn a whole new set of fingerings. To get round this, it was decided to standardise the names of the fingering. This meant that whatever woodwind instrument you learnt, the note played by 3 fingers of the left hand and four fingers on the right would be called a C whether or not it really was a C in concert pitch.
This also makes life easy for somebody to switch between flute (which has a very similar fingering system) and either alto or tenor. As the flute is pitched in C, then this would be yet another set of fingerings to learn were it not for the transposition method of naming the notes.
In order for everyone to play together with no problems, it became the task of the composer or arranger to write music for these different sized instruments in a different key, ie transposed. So we write the music for alto saxophone in a key 6 steps higher than concert pitch. If the music is in the key of Eb concert pitch, we write it in the key of C for the alto saxophone.
Ledger lines
If every note was always written exactly as it sounds, due to the range of the instrument there may be more ledger lines above or below the treble clef. The normal range of the saxophone when written as a transposed instrument has a comfortable three ledger lines above (top F) and one below (low Bb). For the same reason some instruments transpose by an octave or two. A piccolo is pitched one octave above a flute, but the written notes are actually an octave lower than the sounding notes, otherwise the top C would be above five ledger lines. Likewise guitar is written an octave higher than it sounds, so this also is technically a transposing instrument.