In the world of crochet, the names we use are different depending on where you are on the world. Here is a beginner's crochet hook conversion chart. 

If you are learning to crochet or vaguely familiar with it, it may have caused you some confusion when looking at hook sizes. For example, if you are in the UK and the pattern you are using is from the US, it might ask you for a hook size that resembles a letter of the alphabet rather than a number. The best way to get your head around this is by using a crochet hook conversion chart. 

Size really matters when you're learning to crochet. 

Hooks are sized in accordance with the thickness of the shaft. This is important as we need to know which hook suits the yarn that we are using. As you will see in the chart below, the hook sizing is complicated but hopefully this will make things a little bit easier for you.

Measurements  of crochet hooks.

Whilst most of us refer to metric units of measurements, there will still be a vintage pattern or a US pattern that could cause confusion.