Chart 1 of 3
No ace, or an ace that can only count as 1 without busting. Find your total on the left, the dealer's upcard along the top.
Hard 9 through 11 are the classic doubling range — you're betting that one more card gets you to a strong total while the dealer still has to draw at least once more.
Chart 2 of 3
A hand with an ace counting as 11 — written as "A,6" for ace-six, worth 7 or 17. Soft hands can't bust on the next card, which is exactly why so many of them double.
Soft 18 (A,7) is the one hand on this whole chart that changes the most between rule sets — under dealer-stands-on-soft-17, it stands against a 2; under dealer-hits-soft-17, it would double instead.
Chart 3 of 3
Your first two cards match. Splitting turns them into two separate hands, each with its own bet — and since double after split is allowed here, either half can double too.
9,9 is the famous trap: split against 2–6, 8, and 9, but stand against 7, 10, or an ace — an 18 is already good enough there, and splitting risks turning one likely win into two coinflips. 5,5 and 10,10 are never split — they're better played as a hard 10 (double it) and a hard 20 (stand) instead.