If you find that the Fit Kit that came with your QuietComfort Earbuds II does not provide a proper fit, this Alternate Sizing Kit is for you. This kit includes pairs of extra-small and extra-large eartips, and pairs of size 0 and 4 stability bands. A proper fit is critical to ensure the best audio performance and noise cancellation; an eartip with a loose or leaky seal can result in lower performance.