The picture behind the word is a grading system: things are sorted into classes, and first class is the top one. This use takes that picture and puts it on anything you want to praise — a wine, a piece of writing, a person's work. It is a strong word. Good says something is fine; first-class says it belongs at the top of its group, and the comparison with other things of the same type is part of the meaning. It is not the same as perfect: it is about a high standard, not about having no faults. It goes in front of a noun (a first-class hotel) and after be (his writing is first-class). Written with a hyphen when it comes before the noun.

Post offices also sort their work into classes, and this is the quicker, dearer one. In Britain the split is first class and second class, and a first-class stamp usually gets a letter there the next working day. In the United States, First-Class Mail is the ordinary service for letters and light parcels, and it moves faster than the cheaper advertising mail. Either way the word is about the postal service you paid for, not about how good the letter is — a first-class letter can say very little. Taiwan has no exactly matching system, so read it as the faster rate. In front of a noun it takes a hyphen; after a verb you often see it without one.

Here the word is doing its most literal job: travel companies sort their seats and rooms into classes, and this is the top one. A first-class seat is wider, the service comes with it, and the ticket costs a lot more. On planes, first class usually sits above business class rather than beside it, and some airlines now offer only business as their top cabin. Note the spelling: with a hyphen in front of a noun (a first-class ticket, the first-class carriage), and without one when it stands alone as a noun or adverb (a seat in first class, they travel first class).