How is double-strung different from a "normal" harp?
Most harps only have one row of strings. Some have more...
This is a photo of a normal lever harp. You can see a single row of white, red, and blue strings. At the right of the picture, you can see the soundboard, with the darker piece of wood that the strings go into.
A double-strung harp has two rows of strings. The rows, or courses, are exactly the same strings, parallel to each other. Each side has its own set of levers.
In this picture of my Voyageur II➚, on the near side, look at the top, and you can see that the closest string is red. Look to the left of that closest red string. You can see a white and a blue and another white string. These are on the far side of the harp, a second row of strings, same as the first.
When harpers who have never played a double sit down at mine, the first thing they do is look at the placement of their right hand...which means they are looking through two sets of strings. They immediately blink a lot and ask how I play this without getting dizzy.
It took me a week to get used to it, and I've been fine on both single and double ever since.
Why play double?
Your hands don't run into each other. If you've played the "regular" (single-course) harp for a long time, you might not even realize that you make adjustments in your arrangements to prevent your left hand from overlapping notes in your right hand. On a single-course harp, when you are doing a C chord in your left, but your right hand is playing C in the melody, you have to make sure your left thumb doesn't hit your right finger. This doesn't happen on a double-strung. This is the most difficult difference to explain to someone who is not familiar with harps, but it is my number one reason for loving the double-strung.
Easier improvising. With a row of strings for each hand, your hands will never run into each other. Set the levers of each side in a different key for extra fun—one side can be gilss tuning.
Better-sounding repeated notes. For songs with lots of repeated notes, you can split the playing between rows, so you don’t have to dampen the same string to play it again.
Less mid-song lever changing. Set your accidentals on one side so you don’t have to flip in the middle of a song.
More resonance. Doubling the same strings gives a fuller harp sound, because there are more strings to vibrate sympathetically.
Special effects. Echo yourself by playing the same melody with both hands (imagine what bisbigliando sounds like). Play the same notes at the same time for more volume.
Twice the strings in half the space. Take arrangements for a larger harp and move the left hand up an octave.
These harpers have method and tune books for the modern double-strung:
Cynthia Shelhart➚ has a great beginning double-strung method book.
Joanne Griffin➚ has double-strung tune books, including Christmas carols.
Cindy Kleinstuber Blevins➚ has double-strung tune books.