The first important step toward the evolution of the pianoforte was the invention of the keyboard. The origin of this important part of the instrument is uncertain. It is probable that it was first applied to the organ. At any rate, it is said that a keyboard of sixteen keys was attached to an organ built in the Cathedral of Magdeburg at the close of the eleventh century, while most historians date the clavichord or the clavicytherium two centuries later. It is possible, however, that some sort of rude arrangement of keys was employed in the monochord, an instrument used for measuring the scientific intervals between notes of different pitch. It is said to have been invented by Pythagoras in the sixth century before Christ. It consisted of an oblong box with one string stretched across it, and a movable bridge for dividing (or stopping) the string at different points. The continual shifting of the bridge was very troublesome, and as early as the second century (according to Claudius Ptolomæus and Aristides Quintilianus) there was a four-stringed instrument called a helicon. It is surmised that the famous teacher Guido d'Arezzo (born about 995) was the first to use the monochord in teaching singing, and that he devised some kind of a keyboard, because in one of his writings he advises his pupils "to practise the hand in the use of the monochord." The keyboard having been invented, whether for monochord or organ, its application to stringed instruments of the dulcimer or citole family naturally followed. It is impossible to tell whether the first action was a plucking or a striking one, for there are no records, and it is easily conceivable that both may have been used simultaneously in different places. Guido's action is supposed to have consisted of a straight lever with a bridge on the inner end. When the outer end, the key, was struck the bridge arose, gave the string a blow which set it in vibration, and remaining pressed against the string, divided it and determined its pitch. This subsequently became the action of the German clavichord. Another action consisted of a similar straight lever with a piece of quill protruding from the inner end. When the outer end was pressed down the inner end moved past the string and the quill plucked it, causing it to vibrate. This became the action of the clavicytherium, which some writers, without good ground, say was antecedent to the clavichord, and subsequently of the Italian spinet, the harpsichord, and the virginal.