When seed beads were first being manufactured, the standard size bead was referred to as the “null” or “aught” bead. No one knows for sure how the sizes were developed, but it is believed that they were numbered according to how many beads could be lined up side by side in the space of one inch. Accordingly, the larger the number, the smaller the seed bead.
Before starting your beading project, it can be helpful to sort through and take out any beads that may be larger or smaller than the rest. Even in a batch of cylinder beads you will find a couple that are slightly wider or thinner than the rest. You can set these beads aside to use when making increases and decreases in your stitching.
It is also helpful to know that some shaped seed beads such as fringe beads, triangle beads, cubes and bugle beads do not necessarily hold to the aught sizing system. Bugle beads, for instance, are usually sized according to their length in millimeters and noted as a pound sign (#) followed by a number. Cubes and fringe beads are usually just labeled according to their size in millimeters, and triangle beads are either labeled with aught sizes or with a system similar to that of the bugle beads.
Seed bead sizes have changed over time, so seed beads produced today do not necessarily size up according to their number. If in doubt about the actual size of your seed beads, stitch up a sample of the beading stitch you will be using.