In music theory, what distinguishes a Lydian mode from its parallel major scale, and how does this difference influence the melodic and harmonic possibilities?

The Lydian mode is distinguished from its parallel major scale (also known as the Ionian mode) by having a raised fourth degree. A parallel major scale and a Lydian mode share the same tonic, or root note. For example, C Lydian and C Major both start on C. The defining difference is that the Lydian mode has a raised fourth, meaning the fourth note of the scale is a half step higher than in the major scale. In C Lydian, the fourth note is F#, whereas in C Major, the fourth note is F. This single alteration significantly influences both the melodic and harmonic possibilities. Melodically, the raised fourth creates a brighter, more ethereal sound. It avoids the strong pull to the tonic that the perfect fourth (F in C Major) often creates. This allows for more open and less resolved melodic phrases. Harmonically, the raised fourth introduces new chord possibilities and alters the function of existing chords. For example, a major chord built on the second degree of the Lydian mode (in C Lydian, a D major chord: D - F# - A) is a common and characteristic sound. This chord, due to the F#, has a unique quality compared to a D minor chord found in the C Major scale. The raised fourth also makes the tonic chord (C major) sound less stable and more colorful, as the interval between the root (C) and the raised fourth (F#) creates a tritone if inverted, which adds harmonic tension. This gives the Lydian mode its distinctive floating and spacious quality, often used to create a sense of wonder or otherworldly atmosphere.