In Thailand, every table in every restaurant serving noodles (be they a roadside stall or the food hall in an air-conditioned luxury mall), you will find four small jars or pots containing the magic four condiments to sprinkle and splash on your noodles.
Here, clockwise from top left, are sugar (nam dtan/นำ้ตาล), fish sauce (nam pla/นำ้ปลา), dried chilli flakes (prik pon/พริกป่น) and vinegar with chilli (prik nam som/พริกนำ้ส้ม). (Don't get confused by the saucer of chopped chilli - that's for the duck web stew.)
When the noodles arrive have a quick slurp of the soup and then add to taste. The sugar, of course, is sweet, the fish sauce adds salt, the chilli flakes heat and the vinegar adds a little sour hotness. Once added, stir and toss through the noodles to get them to your liking. I add fish sauce, chilli flakes, vinegar and a tiny bit of sugar. My wife adds a good spoon of sugar, fish sauce, a generous pile of chilli flakes and a lot of vinegar. Everyone is different!