What Clueless said (his username is a lie, don't let him convince you otherwise), plus the context that, while single-piece chucks (the machine's taper and the chuck body are machined from one solid piece of metal) do exist, a separate arbor as he posted is much more common. Thus the arbors have two specifications. In my case I got a 2MT - 33 JT for my lathe, and a 33 JT chuck (both from Shars, through their Discount Machine Shop eBay store). I put them together the day I bought them and they have never come apart since then.