their right-hand sides. Piano accordions use a piano-style musical keyboard; button accordions use a buttonboard. Button accordions are furthermore differentiated by their usage of a chromatic or diatonic buttonboard for the right-hand side. Accordions may be either bisonoric, producing different pitches depending on the direction of bellows movement, or unisonoric, producing the same pitch in both directions. Piano accordions are unisonoric. Chromatic button accordions also tend to be unisonoric, while diatonic button accordions tend to be bisonoric, though notable exceptions exist. Accordion size is not standardized, and may vary significantly from model to model. Accordions vary not only in their dimensions and weight, but also in number of buttons or keys present in the right- and left-hand manuals. For example, piano accordions may have as few as 8 bass buttons (two rows of four notes), or up to 120 (six rows of twenty notes) or beyond. Accordions also vary by their available registers and by their specific tuning and voicing. Despite these differences, all accordions share a number of common components. Universal components Bellows The bellows is the most recognizable part of the instrument, and the primary means of articulation. The production of sound in an accordion is in direct proportion to the motion of the bellows by the player. In a sense, the role of the bellows can be compared to the role of moving a violin's bow on bowed strings. For a more direct analogy, the bellows can be compared to the role of breathing for a singer. The bellows is located between the right- and left-hand manuals, and is made from pleated layers of cloth and cardboard, with added leather and metal. It is used to create pressure and vacuum, driving air across the internal reeds and producing sound by their vibrations, applied pressure increases the volume. The keyboard touch is not expressive and does not affect dynamics: all expression is effected through the bellows. Bellows effects include: Volume control, including swells and fades Repeated short, rapid changes of direction ("bellows shake"), which has been popularized by musicians such as Renato Borghetti (gaucho music) and Luiz Gonzaga, and extensively used in Forró, called resfulego in Brazil Constant bellows motion while applying pressure at intervals Constant bellows motion to produce clear tones with no resonance Subtly changing the intonation to mimic the expressiveness of a singer Using the bellows with the silent air button gives the sound of