With major scales, the distance between the root note and degree 6 of the scale is a major sixth, so in the case of C, a major sixth above would be A. Jump up from C to A in the course of a melody ...

Play these notes starting on C and you're playing the C major scale (also known as the Ionian mode – the 'first' mode). Start on D and you're playing the second mode (D Dorian, as it happens).