If a creature has damage resistance, and is dealt an odd amount of damage, do you round the final damage up or down? The Player's Handbook, annoyingly, only gives the example of 20 ÷ 2 = 10. If it had been 21 ÷ 2, would that round to 10 or 11? If the creature is dealt 1, does that round to 0 or 1?
You always round down unless told otherwise.
In the very beginning of the Player's Handbook it lays down some ground rules for everything that follows. Along with the "how dice are used" and "specific overrides general" stuff, it includes a subsection entitled "Round Down" (p. 7):
There’s one more general rule you need to know at the outset. Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.
So half of 11 damage would be 5, and half of 1 damage would be none.
There is some information that deviates away from this. In the Player's Handbook (PHB), it has two examples of rounding both on page 7. The first, under "Game Dice" says:
To simulate the roll of 1d3, roll a d6 and divide the number rolled by 2 (round up).
And later, under "Round Down":
There's one more general rule you need to know at the outset. Whenever you divide a number in game, round down if you end up with a fraction, even if the fraction is one-half or greater.
And sometimes, for whatever it is you are calculating, it may say either.