Hummel's influence can also be seen in the early works of Frédéric Chopin and Robert Schumann, and the shadow of Hummel's Piano Concerto in B minor as well as his Piano Concerto in A minor can be particularly perceived in Chopin's concertos. This is unsurprising, considering that Chopin must have heard Hummel on one of the latter's concert tours to Poland and Russia, and that Chopin kept Hummel's piano concertos in his active repertoire. Harold C. Schonberg, in "The Great Pianists", writes "...the openings of the Hummel A minor and Chopin E minor concertos are too close to be coincidental". In relation to Chopin's Preludes, Op. 28, Schonberg says: "It also is hard to escape the notion that Chopin was very familiar with Hummel's now-forgotten Op. 67, composed in 1815 – a set of twenty-four preludes in all major and minor keys, starting with C major".