Cats often purr when they are happy and contented.  In a cat’s voice box, or larynx, are two sets of vocal cords.  One set of cords is above the other and each set produces different sounds.

The lower are the true vocal cords. The cat uses the true cords to produce the “meow” and the other ordinary sounds it makes.  Above these are membranes called the “false” cords.

When a cat is happy, it allows the upper cords to vibrate as the air passes in and out of its lungs.  The vibrating membranes produce a soft fluttering sound which is called purring. – Dick Rogers

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