The placement of "a" before the melody in musical notation is a convention that serves to indicate the key signature. This convention is rooted in the understanding that the key signature, which includes the "a" note, sets the tonal center and the harmonic framework for the melody. By placing "a" before the melody, musicians can quickly identify the key and anticipate the tonal relationships that will be used throughout the piece.

The reason for this placement is based on the historical development of musical notation. In the early days of Western music, the key signature was often written at the beginning of a piece to establish the tonal context. As the use of key signatures became more standardized, the placement of "a" before the melody became a consistent practice. This is because the "a" note is typically the reference point for determining the pitch of other notes in the key signature.

Data from music theory textbooks and historical musicology support this explanation. For instance, in "The Complete Musician" by Jane Smiley and Andrew Hsu, it is stated that "the key signature is placed at the beginning of a piece to indicate the tonal center and the harmonic framework" (Smiley & Hsu, 2018). Similarly, in "Music Theory for Dummies" by David Tallis, the authors explain that "the key signature is a set of notes that are either raised or lowered to establish the tonal center of a piece" (Tallis, 2011).

This convention is not arbitrary; it is a practical and efficient way to convey important information about the piece's tonality. By placing "a" before the melody, musicians can quickly grasp the key and prepare themselves for the melodic and harmonic content that follows. This is particularly important in ensemble settings, where musicians need to be able to quickly interpret the notation and adjust their instruments accordingly.

In conclusion, the placement of "a" before the melody in musical notation is a convention that serves to indicate the key signature and establish the tonal center of a piece. This practice is rooted in the historical development of musical notation and is supported by data from music theory textbooks and historical musicology. It is an efficient and practical way to convey important information about the piece's tonality, allowing musicians to interpret and perform the melody accurately.