The most important consideration in mask selection is fit. No two faces are alike, and the SCUBAPRO broad range takes individual differences into account. The basic difference in masks lies in the number of lenses. Single-lens masks were the first developed and remain a popular choice worldwide because they provide a broad, uninterrupted view. With the advent of silicone skirts, twin- lens masks have become increasingly popular because they typically lower internal volume, making clearing easier.