The true hammer dulcimer is a close relative to the psaltery, the chief difference being that the psaltery is usually plucked and the dulcimer is usually struck. Early varieties were rather simple, having relatively few strings which passed over bridges only at the sides. The versatility of the dulcimer was greatly increased by clever placement of additional bridges. This results in two notes from each string in the ratio of a perfect fifth interval. Other ratios have occasionally been used.
Many dulcimers have another bridge added near the right side to carry bass courses.
The bass courses pass high over the bass bridge and low through holes or interruptions in the treble bridge. Likewise, the treble strings are raised at the treble bridge and pass low through the bass bridge. Thus, the treble strings may be struck near the treble bridge without danger of hitting bass strings, and bass courses can be played near the bass bridge without running afoul of treble strings. This arrangement triples the number of notes possible without any increase of size or consequent increase in distance from the player. Dulcimers of this sort began appearing in Europe during the 16th century and remained rather popular to the 18th.