Hooks and needle manufacturers create gauges for their own products and they often label their hooks and needles differently, they don’t do this to confuse you, they do this because the standards in their own country were defined and our communities weren’t global as they are now. Traditionally, the Japanese hook sizes were different to the US hook sizes and the British had their own system again. Hooks and needles have always been made from a variety of materials, some from bone, wood, metal, aluminum and recently from bamboo and resins. Each material behaves differently when manufactured or created. For these reasons manufacturers often created their own sizing gauges, this way you could identify their hooks and needles to the size marking they intended. If you tried measuring another manufacturers hook or needle with their gauge they could well be marked with a different letter/number than the gauge would indicate.
In the US crochet hook labels may sometimes have more than one letter such as M/N or P/Q this is because over time the labelling varied too. To try to eliminate the confusion between the various number, letters and size notations given to crochet hooks and knitting needles the Craft Yarn Council has worked with hook and needle manufacturers who have agreed to make metric (millimeter/mm) sizing more prominent on packaging. This doesn’t help you much if you have older hooks, with old labeling or hooks with no markings at all. You could use calipers to accurately size your hooks and needles, however they’re not very practical in your yarn and project bags. Therefore, using a hook and needle sizing gauge that’s not manufacturer dependent, one like DualGauge, can be very useful to own. Use it together with a conversion chart (below) if a pattern you’re using still refers to the hooks or needles in something other than mm sizing.
Below is a conversion chart derived from the one published by the Craft Yarn Council (http://www.craftyarncouncil.com/hooks.html).