The Psaltery

There were three categories of musical instruments in the Middle Ages and early modern period - wind, string, and percussion. The Psaltery was an instrument which was a cross between a harp and lyre. It is a kind of a lyre or a harp with twelve strings, but having a trapezodial (table-like) sounding board under the strings. 

The archetype of the instrument carried horizontally and struck with two sticks, found in iconographical documents in ancient Babylon (1600-911 BCE) and neo-Assyria (911-612 BCE).

The instrument was brought to Europe by the Arabs through North Africa and Spain during the Middle Ages and also to China where it was referred to as the "foreign qin". It is the main instrument used in the classical Magam tradition along with the Iraqi spike fiddle "joza"

The instrument can also be placed on a table and plucked by fingers or with  plectrum or quill.

The psaltery (psalterion, saltere, sauterie, Psalterium, Psalter, salterio) is an ancient instrument seen in many forms. Developed in the Middle East, the psaltery is in the family of chordophones. Vibrating strings by running a bow across them makes their sound. The psaltery's strings run the entire length of the instrument and put it in the same classification as the zither.

The psaltery was a very important instrument during the Medieval Period. The name of psaltery entered Christian literature in the 3rd century B.C. translation of the Old Testament called the Septuagint where, in the Psalms, "nebel" was translated "psalterion". The book of Psalms has also become known as the Psalter (or psalterium), from the hymns sung with this harp.

Southern Europe, influenced by Moorish Spain, preferred the trapezoidal psaltry with three or four strings to a note. Northern psalteries tended to be triangular or wing-shaped and single or double-strung. Like most other instruments of the time, the psaltery had no specific repertory, but was used to play whatever music the occasion demanded. It was referred to frequently in lists of musicians and instruments and in the art of the time. The psaltery was widely used until about 1500, but could not cope well with the chromaticism of the Renaissance, so was used less as time passed.

Illustrations from the 12th century onwards depict a number of different forms of the instrument. Such plucked psalteries were well known throughout Europe during the Middle Ages: Geoffrey Chaucer (c.1340 - 1400) refers to it in his Millere's Tale. By the 18th century it had developed into several other instruments, including the hammered dulcimer. The harpsichord is a hammered dulcimer with a keyboard mechanism; which eventually gave rise to the most important instrument of modern times, the piano. Historic illustrations show the plucked psaltery held against the chest with the narrow, pig-snout end pointed down, or resting on the lap. 

The earliest psalteries had gut strings. Later steel strings were added for a louder, brighter sound. Illustrations from the 12th century onwards depict a number of different forms of the instrument. Such plucked psalteries were well known throughout Europe during the Middle Ages: Geoffrey Chaucer (c.1340 - 1400) refers to it in his Millere's Tale. By the 18th century it had developed into several other instruments, including the hammered dulcimer. The harpsichord is a hammered dulcimer with a keyboard mechanism; which eventually gave rise to the most important instrument of modern times, the piano. Historic illustrations show the plucked psaltery held against the chest with the narrow, pig-snout end pointed down, or resting on the lap. The strings of the plucked psaltery are plucked, either with fingers or with a quill or plectrum.

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16th century diatonic French Psaltery 

sources: http://singingtreeinstruments.info/2.html

http://www.diabolus.org/guide/psaltery.htm

http://musicoutfitters.com/ethnic/psaltery.htm