You will hear us referring to fill power on our down duvets. It is the measurement of the amount of space that one ounce of down will occupy, when it's allowed to reach its maximum loft (or fluffiness as we like to call it). The higher the fill power, the larger the clusters of down, and the better the quality. Mature birds will produce the biggest down clusters, and as they are harder to come by, this adds to the price. Any fill power over 600 is known to be a premium quality, and ours range from 650 to 800.