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In page John Serry Sr.:

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As an educator, Serry also published several method books for his elementary, intermediate, and advanced grade students between 1945 and 1983.[11] He took note of the limitations imposed by the Stradella bass system during performances of classical music. In an effort to circumvent these limitations, he designed and developed a working model of a free-bass system for the accordion during this decade. It incorporated dual keyboards for the soloist's left hand while incorporating two sets of reeds which were tuned in octaves. This gave the soloist access to a range of tones which exceeded three and one-half octaves.[57]