In a scale, each note has a number which is written at the bottom of the measure, under the note. For example:

A         B        C#        D        E       F#           G#
 I          ii        iii         IV       V      vi            vii*


These symbols are the roots of triads or chords.  But they are used in transposing one piece that is written in one scale  eg. A major  to a different scale. Lets go to B Flat major scale.

B flat major scale has two flats-B and E

Bb       C       D       Eb       F         G        A
  I         ii      iii        IV       V         vi      vii*   major, minor, minor, major, major, minor, leading tone-this succession of chords applies to the major scales only

In a Bb major scale  the first note c# in the A scale becomes D.  C# is iii in the A major scale but iii is D in the Bb major scale.

Bb  C  D  Eb  F  G  A

                                                                    A   B  C#  D  E  F#  G#