They can have as many as 47 and as few as 28 for a traditional harp (like in orchestras). Smaller Celtic harps have fewer strings.
A redd (single reed) is used on clarinets and saxes. Double reeds are used on oboes and bassons. they are called double because there are 2 peices of reed cane instead of one. They take a longer piece of reed cane and fold it over and tie it. Then they cut off the reed where is doubled over and that makes 2 reeds tied together.
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