Early Harps
Here are photos and brief descriptions of some of the types of harp that have been prominent in musical history.

Gothic style Renaissance  Harp

These small harps were the standard harp throughout Europe in the late Middle Ages and the Renaissance period in music history. They were strung with gut strings at a much lower tension than  we are accustomed to today. Earlier models had 19 to 22 strings, later harps were larger and had 26 to 30 strings. They were tuned  diatonically, half steps usually accomplished by pressing the  string against the neck near the top. Current scholarship tells  us that these harps almost always employed brays, small L shaped  pegs that touch the string very lightly near the soundboard. The effect is a nasal rattling sound similar to a crumhorn or  regal organ. The soundbox was generally hollowed from a plank  of hardwood, giving the harp a distinctive plucked sound that  differed from the lute, an instrument it often played with in consort.

Italian Arpa Doppia

This three-rank Italian  harp of the late Renaissance or early Baroque period had 26 chromatic strings in the central row, with 24 trebles on the right, and 25 basses on the left. This gives a two octave diatonic duplication in the middle range of the harp. The center row of strings supplies all of the half steps, including the enharmonic notes. The range descends to GG (one and a half octaves below middle C). The soundbox is built up of hardwood staves, and the soundboard is maple, as were all but the latest examples.

Early Spanish Harps

The Spanish single rank and cross-strung harps generally had 29 diatonic strings, lowest  note C, two octaves below middle C. The chromatic strings  intersected, or went between, the diatonic strings, forming an X. In this way, all chromatic and diatonic strings were accessible to both hands at any point on the harp. The soundboard is very guitar-like, spruce with cross bracing, producing  a marvelous sound.

The single rank harp could be fretted against the neck for half steps.

Wire strung  harps

Many would say that the ancient Irish harps are  the predecessors of our current lever harps in spirit, if not in technical features. These instruments were wire strung (brass, iron, silver, or gold) often with the soundboxes carved from a single piece of willow (bog wood).