These are Floss-Paks, designed to hold thread skeins. They are snap lock bags 7.6 x 13 cm in size and have a panel on the front to write the colour number. The bags can then be attached to an easy-opening storage ring. Note that that only the 15 bag starter pak come with a storage ring, but we do sell these separately, see here. You can fit as many bags onto the ring as you want, so you can hold all you need for your current project together while other bags with threads that you don’t need can be stored away. Great for travel as bags & ring are very compact.