In blackjack, unwritten rules about splitting and doubling down often revolve around player etiquette and optimal strategy. Players generally split pairs to maximize potential winnings, especially with aces and eights, but may avoid splitting tens or face cards as the total is strong. Doubling down is typically recommended when the player has a total of 10 or 11 against a dealer's weaker upcard, allowing for a chance to increase winnings with one additional card.