The hammered dulcimer is a trapezoidal wooden box with horizontal wire strings across its top, usually in double courses (groups of two strings close together, tuned to the same note). The strings are struck with two light-weight, hand-held "hammers": most often wooden sticks but sometimes with shafts of other materials. Various materials (e.g., bare wood, leather, felt) are used on the striking surfaces of the hammers to create a variety of tonal qualities, from bright and percussive to soft and harp-like. Basic instruments have two vertical bridges across which the strings cross, creating three striking areas. String courses are arranged "diatonically" up and down most of the instrument: Striking adjacent string courses produces major, minor, or modal eight-note scales without intervening chromatic notes.

Hammered dulcimers vary in size, range, string spacing, type of woods used, tone quality, and ornateness. An instrument's basic configuration is designated by the number of string courses crossing each bridge. For example, the instrument in the top picture in the left column is a 12/11 "student" model by James Jones, and is the instrument on which I learned and played for the first 4 years.  It has the range of a violin (2-1/2 octaves), medium (1") string spacing, a bright, non-resonant tone, and works best for playing fiddle tune melodies. Common intermediate sizes, better-suited to playing self-accompanied Celtic airs, are 15/14 and 16/15.  Larger instruments have the missing chromatic notes available on the periphery of the instrument, and often have a third "chromatic" or "super-bass" bridge to include some of those notes as well as to expand the range farther down into the bass.  The instrument shown in the bottom picture is a Songbird Warbler 17/16/8, which I have played since 2002 (dampers, to create a percussive sound when desired, were added later by Rick Fogel). It is moderately large, with medium (1-1/16") string spacing, a 3-3/4-octave range, moderate resonance, and works reasonably well with a wide range of musical styles. 

The hammered dulcimer can be thought of as a primitive or rustic piano: Both instruments make sound via wire strings being struck with wooden "hammers." While pianos have 88 mechanically-operated hammers, dulcimer players use two hand-held hammers. The natural sustain of the instrument compensates for the limitation of only being able to sound two notes at the same time. Unlike the piano, the higher-pitched melody notes are to the left side of the hammered dulcimer. Because of this, the hammered dulcimer may be unique, or at least unusual, in that left-handers are not at an inherent disadvantage: If anything, they may have a slight edge over right-handers. 

Playing the hammered dulcimer is a largely visual endeavor. Due to the lack of direct tactile localizing feedback when striking the strings with the hammers, players usually need to look at the instrument while playing: It is difficult to sustain accurate playing while continuously reading music. The ability to read music is a very useful skill for learning new tunes, but most players then memorize their repertoire or play somewhat "by ear." Those with backgrounds improvising or playing by ear on other instuments, such as guitar, generally learn the hammered dulcimer readily. Percussionists also make excellent hammered dulcimer players.