Wire comes in three different forms of hardness: dead soft, half-hard, and full-hard. Below are some basic definitions for each type:
Dead Soft: As the term indicates, this wire is very soft and can be bent with your hands. It is often used for wire-sculpted jewelry.
Half-Hard: It is harder than dead soft wire because it has been pulled through a draw plate (a tool with holes in it the same size and shape of the wire).
Full-Hard:This wire is harder than half-hard wire because it has been pulled through a draw plate more times than half-hard wire.
Wire comes in a variety of shapes such as round, square, and half-round. In most (though not all) of the wire projects I post on this site. I use round wire, and it tends to be the most versatile. Artists who prefer to make wire wrapped style jewelry usually use a lot of square and half-round wire. They’ll use the half-round to wrap around the square when attaching the wires together.
In the US, the size or thickness of wire is measured in gauge (also spelled gage) while in most European countries they measure it in millimeters. Below is a list of the different sizes and what I normally use each size for along with some other helpful information. The Rio Grande gems and findings catalog provides measurements in gauge, inches, and millimeters, and I used it for some of the following information.
26 and 24 gauge (.40mm - .50mm) – This size is good for beads that have small holes in them such as pearls. I usually buy this in half-hard, and I like to use if to make pearl and wire bracelets by creating a bead and wire chain. In an ounce of 26 gauge wire there is about 76 feet and in 24 gauge there is about 48 feet of wire.
22 and 21 gauge (.65mm - .71mm) – I use these sizes a lot, though not many vendors seem to carry 21 gauge. These are both very versatile sizes because they are pretty thick but most beads (like crystals and stone beads) can fit on them. I buy this normally in dead soft, and I use these sizes to make bead and wire chains and also for a variety of jewelry findings, which I often make like ear wires or head pins. I sometimes make clasps with these sizes as well but only if the piece isn’t too heavy. One ounce of 22 gauge has about 31 feet and 21 gauge has about 24 feet of wire.
20 gauge (.80mm) – This is about the thickest wire I use for the most part. It is good for making clasps because it’s still pretty easy to work with but is strong as well. I normally buy this in dead soft. You can get about 19 feet of 20 gauge wire when you buy it by the ounce.
This is by no means a complete list of sizes, but it will give you a good place to start. As I said, it is up to you to find your own wire jewelry path, but I hope this information has at least started you on your journey.