Music historians often refer to the three B's of classical music: Bach, Beethoven and, Brahms. Their compositional lives cover 250 years from 1650-1900.
Out of so many great composers why are these three singled out? Each represents the culmination of three specific periods of classical music: Bach = Baroque, Beethoven = Classical, and Brahms = Romantic. Each fully explored and expanded the musical styles of their time and anticipated and influenced the direction of classical music to follow.
Bach was an easy choice for the first B. With the development of "equal temperment" for keyboard instruments (each note was tuned an equal distance apart allowing keyboard instruments to be played in every key), the major and minor keys were firmly established as the primary modes or scales on which music is based. Bach fully explored this concept of key relationships. He wrote "48 Preludes and Fugues for the Well-Tempered Clavier" - one Prelude and accompanying Fugue in each of the 12 keys and liked it so much he did it twice.
Contrapuntal music, where the voices have overlapping melodies, was the predominant compositional style of the earlier Renaissance period. Bach elevated counterpoint to its ultimate realization particularly in his keyboard works, such as The Goldberg Variations.
At the same time, Bach also further developed the concept of homophony, in which one melody is supported by harmonies or chords. Through his hundreds of chorales (hymns), he devised a system of chordal progressions that has lasted to the present day. All students of music theory today learn to apply Bach's "rules" of harmony that have been determined by the analysis and deconstruction of his chorales which are amazingly consistent in harmonic construction and chordal progression. When faced with harmonizing a melody, every student has to answer the question, "What would Bach do"?
Bach was a teacher.
The Classical period of music is dominated composers Haydn, Mozart and Beethoven and the development of the structure of music of musical forms. Everything got bigger : orchestras and operas expanded in size in terms of the numbers of instruments and singers involved. The sonata cycle, a multi-movement form became a predominant large form for everything from solo instrumental works to chamber music and symphonies. The major/minor tonality of Bach was expanded with new harmonic structures creating an exciting emotional vocabulary for composers.
Beethoven found some elements of the sonata cycle too confining for what he wanted to express. Instead of the fast-slow-fast order of movements, he inserted introductions to the beginning, changed the order of movements, sometimes blended movements one into another without any delineation, and introduced a choral movement into an instrumental form. He brought a new type of emotionalism and passion into music. Beethoven expressed the human condition, no matter how dismal or grief-stricken, and always ended with triumph and joy.
Beethoven was a disruptor.
Brahms had some competition to become the third B; strong contenders were Berlioz and von Bulow. Brahms prevailed because of his influence on future composers.
Brahms felt the weight of what had come before. He famously said that after Beethoven's 9th Symphony, there was nothing more to be said in that genre. It was twenty years before Brahms composed his first symphony. By then, he had established his own musical vocabulary and innovations writing in only absolute style as opposed to some of his contemporaries programmatic or story-telling style.
Brahms often interlaces his ingenious harmonies with elements of contrapuntal style. His music is multi-layered and emotional. His approach to tonality was unique, stretching it to its limits, anticipating the impressionists Debussy and Ravel. The 20th century composer, Arnold Schoenberg, a leader in Expressionism and atonality, considered Brahms to be his most influential predecessor.
Brahms was an influencer.
Classical music is has constantly evolved over centuries. Young composers learn from those who came before and create their own unique visions on that firm foundation.