What is a musical key?
When we speak of a musical key we are referring to a much larger concept than just a scale or a key tone (tonic). Of course, a musical key includes a tonic and a scale (mode), but the psychological magic of a musical key is not found in memorizing scales and chords, it is found in discovering the family relationships between all the constituent parts of a chosen key. For example, the harmonic qualities of chords, chord changes, intervallic movement in melody, and predictable tonal behaviors used in improvisation are all implied by the key.
A musical key describes a context for music; it is a structural system that creates meaning for any given note or set of notes used in the music –a palette of related tonal materials used for emotional communication. Composing within a musical key creates a context in which to measure movement, to create the sense of a musical journey, a point of departure for materials that are not in the key, and a sense of coming home at the completion of a journey. It is this movement within a specified key that creates emotional communication and connection.
Musical key as a system
A musical key can also be figured as a solar system, of sorts, where the tonic is the gravitational center of the key. The tones supplied by the scale are the building blocks for melody and harmony (chords) within the key, where each tonal degree of the scale occupies a fixed intervallic distance away from the key center (tonic). When you understand the cosmos of the key, you understand how the movement of melodic tones, intervals, scale degrees, and chord tones relate and behave with each other.
Melody
Melodic tones travel in various tension and release movements in relation to their distance from the key note. Melodic tones are derived from the scale of the key and live inside of chords.
Chords
Chords are little subsystems inside the larger key and have their own tonal centers inside the smaller chord structure. Each voice in the chord orbits around the central chord tone (chord root), while each chord system has an orbital relationship to the larger key center (tonic).
In short
A key is the tonality central to a piece of music. In other words, a key is based on a tone and a structure built around that tone. It is this structure that provides context and meaning inside the music.