What is basic strategy?
Basic strategy is defined as playing your present card combination as well as you can given only the knowledge of dealer's up card and the rules of the game. Basic strategy does not consider any cards that may have been dealt prior to the current deal, if any. It does consider all cards in the player's hand along with the dealer's up card. In other words it is assuming all hands are dealt from a full shoe and it only considers knowledge of a single player's cards plus dealer's up card.
Composition dependent basic strategy
Composition dependent strategy considers each and every card in player's hand in determining a strategy. With comp dependent strategy hands that evaluate to the same total will not necessarily be played the same. For example when dealt from a full single deck a hand of 10-5 v 10 will be hit and 5-5-5 v 10 will stand if played according to CD basic strategy even though both hands total 15. However, when dealt from 2 full decks both 10-5 and 5-5-5 versus an up card of 10 would be hit if played according to correct CD basic strategy. In the case of a single deck removing 3 fives and one 10 is enough to make standing on 15 better than hitting while hitting on 15 is better than standing when 2 tens and 1 five are removed. There are more cards to start with in a double deck and hitting is better than standing in either case. In general as more decks are added then all hands that evaluate to the same total become virtually equivalent. This is because the cards comprising the hand become more and more insignificant as more and more cards (decks) are added.
Practical approach to basic strategy (1) ?
The best possible basic strategy would be CD basic so let's consider what it may take to learn it. There are 3072 possible player hands of 2 or more cards, numerous common rules variations, each hand has potentially a different strategy versus each up card, and number of decks are commonly 1, 2, 4, 6, or 8 with possibly differing strategies. Out of the 3072 hands there are some that could be eliminated. For example a hand of 16 aces would never be encountered unless several prior misplays were made. First a hand of 16 aces is not even possible if decks are less than 4. Next initial 2 cards will have been A-A and almost always should be split. If player didn't split and continued drawing aces then at some point he would have 9, 10, 11 aces - hands of soft 19, 20, 21 on which stand is virtually always best. The point is that if one wanted to learn perfect CD basic, this hand could be eliminated. However, there would still be plenty of hands left to consider. There would be less for single deck and more for more decks. The Composition Dependent Combinatorial Analyzer on this site could be used get the right basic strategy for any combination of the above (even including the 16 ace hand.) In order to be able to adapt to any common number of decks and common rules variations you would need to remember a mountain of data. Just to record relevant data would be a big task, let alone to remember it.
Practical approach to basic strategy (2) ?
OK basic strategy approach #1 above might give the best expected value but it isn't simple enough, so what can we do instead? We want to eliminate as many intricacies as we can without giving up too much in expected value in the process. How about simply grouping player hands by their hard or soft totals and then playing each total the same way dependent upon dealer's up card? A pair that is split and/or resplit will also be played according to its new total after splitting. It turns out that this method called total dependent basic strategy approximates composition dependent basic strategy pretty well and is much simpler. There are some minor variations in TD basic depending on number of decks. Total dependent basic strategy for differing rules and number of decks can be found at this link.
Practical approach to basic strategy (3) ?
Total dependent basic strategy, above, strikes a good balance between practicality and attaining a reasonable approximation of best possible basic strategy expected value. Can we make any further simplification? It turns out that we can eliminate number of decks as a variable in our basic strategy without much loss in expected value, which means we can learn one basic strategy for a given set of rules for any number of decks. If we input a large number of decks, say 10000000, into the cdca program on this website then the resultant best strategy will be independent of hand composition and be 100% dependent upon hand total because the cards that go into the play of a hand are insignficant in changing a shoe's probabilities when dealt from a 10000000 deck full shoe. Also we'll apply this strategy to any number of decks. We'll call the resulting strategy Generic basic strategy, meaning we will apply it as universally as we can as our base approximation of best basic strategy.
Below are 6 versions of Generic basic. They are mainly the same but have some differences because each applies to different rules variations. The basic strategies listed in them will mostly be right for any number of decks. Additional plays for a specific condition such as doubling A-2 v 5 in single deck rather than hitting can be used at player's convenience as exceptions to Generic Basic. The more decks there are the closer Generic Basic is to best basic strategy. Below EVs for comparison are for rules double any 2 cards, no double after split, no hitting split aces (except where noted,) no surrender allowed, BJ pays 3 to 2. CDEV is comp dependent basic strategy EV. TDEV is total dependent basic strategy EV that incoporates differences due to number of decks. GBEV is Generic Basic strategy EV from above, which plays all number of decks using the same total dependent strategy. The main difference between GBEV and TDEV is GBEV strategy does not double a small number of relatively close decisions whereas TDEV strategy does.
S17 full peek - Full peek, dealer stands on soft 17
1 Deck ---- CDEV: +.0243% ---- TDEV: -.0147% ---- GBEV: -.0487%
2 Decks ---- CDEV: -.3485% ---- TDEV: -.3624% ---- GBEV: -.3677%
4 Decks ---- CDEV: -.5219% ---- TDEV: -.5274% ---- GBEV: -.5285%
6 Decks ---- CDEV: -.5788% ---- TDEV: -.5819% ---- GBEV: -.5823%
8 Decks ---- CDEV: -.6072% ---- TDEV: -.6091% ---- GBEV: -.6093%
H17 full peek - Full peeks, dealer hits soft 17
1 Deck ---- CDEV: -.1688% ---- TDEV: -.2051% ---- GBEV: -.2261%
2 Decks ---- CDEV: -.5521% ---- TDEV: -.5633% ---- GBEV: -.5672%
4 Decks ---- CDEV: -.7336% ---- TDEV: -.7382% ---- GBEV: -.7389%
6 Decks ---- CDEV: -.7932% ---- TDEV: -.7960% ---- GBEV: -.7963%
8 Decks ---- CDEV: -.8230% ---- TDEV: -.8249% ---- GBEV: -.8250%
S17 no peek - No peek, dealer stands on soft 17
Hitting split aces is not allowed
1 Deck ---- CDEV: -.0809% ---- TDEV: -.1241% ---- GBEV: -.1469%
2 Decks ---- CDEV: -.4526% ---- TDEV: -.4678% ---- GBEV: -.4723%
4 Decks ---- CDEV: -.6284% ---- TDEV: -.6348% ---- GBEV: -.6359%
6 Decks ---- CDEV: -.6865% ---- TDEV: -.6900% ---- GBEV: -.6905%
8 Decks ---- CDEV: -.7154% ---- TDEV: -.7177% ---- GBEV: -.7179%
Hitting split aces is allowed
1 Deck ---- CDEV: +.0477% ---- TDEV: +.0041% ---- GBEV: -.0187%
2 Decks ---- CDEV: -.2981% ---- TDEV: -.3135% ---- GBEV: -.3180%
4 Decks ---- CDEV: -.4607% ---- TDEV: -.4671% ---- GBEV: -.4683%
6 Decks ---- CDEV: -.5143% ---- TDEV: -.5180% ---- GBEV: -.5185%
8 Decks ---- CDEV: -.5411% ---- TDEV: -.5434% ---- GBEV: -.5436%
H17 no peek - No peek, dealer hits soft 17
Hitting split aces is not allowed
1 Deck ---- CDEV: -.2796% ---- TDEV: -.3204% ---- GBEV: -.3414%
2 Decks ---- CDEV: -.6618% ---- TDEV: -.6748% ---- GBEV: -.6788%
4 Decks ---- CDEV: -.8424% ---- TDEV: -.8478% ---- GBEV: -.8486%
6 Decks ---- CDEV: -.9017% ---- TDEV: -.9050% ---- GBEV: -.9053%
8 Decks ---- CDEV: -.9314% ---- TDEV: -.9337% ---- GBEV: -.9337%
Hitting split aces is allowed
1 Deck ---- CDEV: -.1519% ---- TDEV: -.1931% ---- GBEV: -.2141%
2 Decks ---- CDEV: -.5090% ---- TDEV: -.5222% ---- GBEV: -.5261%
4 Decks ---- CDEV: -.6768% ---- TDEV: -.6823% ---- GBEV: -.6831%
6 Decks ---- CDEV: -.7319% ---- TDEV: -.7352% ---- GBEV: -.7355%
8 Decks ---- CDEV: -.7594% ---- TDEV: -.7617% ---- GBEV: -.7618%