Complex sentence: In it's most developed form, the vihuela was a guitar-like instrument with six double strings (paired courses) made of gut, tuned almost like a modern guitar, with the exception of the third string, which was tuned a semitone lower.
Split into simpler sentences: In it's most developed form, the vihuela was a guitar-like instrument with six double-strings (paired courses) made of gut. Vihuelas were tuned almost like a modern guitar, with the exception of the third string, which was tuned a semitone lower.