The newest of the series, the M5 is a slimmer version of our best-selling M8. This 18/8 stainless steel, single wall bottle’s new sleek and lightweight design makes it functional for stashing in small places on the go, like purses or back pockets, while its smaller frame offers a petite and more sophisticated twist to a well-loved favorite. Note: Single Wall is so lightweight because there is no double wall to weigh it down. No double wall means no insulation, folks. Cold beverages only in this one.