Barett, there is an easy way to tell whether your str8 is nut or not. Recognizing on board takes some practice.
Below are all possible str8s with 45678 that use BOTH your cards.
The red cards are yours, and the black are on the board.
The blue cards are possible cards for one of your opponents that beat your str8.
456
78 - nut
45
678 - nut
4
567
8 - nut
4567
8 - 56789 is better
45
678 - nut
4
56
78 - nut
456
78 - 56
78
9 is better
4
5678 - nut
45
678 - 5
678
9 is better
45678 - 678
9T is better
As you can see, IF the lowest card of the str8 is yours, then you don't have the nut str8. And vice versa, if the lowest card of the str8 is on the board, you DO have the nut str8.
What about str8s where only one of your cards is used? Here are all possibilities:
4567
8 - 56789 is better
456
78 - 56
789 is better
45
678 - 5
678
9 is better
4
5678 -
5678
9 and 6789T are better
45678 - 5678
9 and 6789T are better
As you can see, there is no way to have the nut str8 if only one of your cards is used UNLESS it is A in A-high str8. The reason should be clear from the list above. A better str8 always involves an overcard to the board cards. Obviously, if the board is TJQK and you hold the
A for an A high str8 TJQK
A, then it is the nuts with one card (which is the ONLY exception).
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Another way to think about str8s that might help is to imagine str8s as a fork with 5 prongs. Two of the prongs are your cards. To make a str8, you need to "match" the other 3 prongs of the fork to the board.
Consider, for example, that you hold 48. There is only one way to make a str8 with both cards, namely to have on the board 567. From the examples above, you can see that it can never be the nut str8 since the low card of the str8 is yours. This is one reason to not draw to such str8s. The other is that there is ONLY one way to make a str8 with 48 using both cards.
Let's now list all possible connectors and imagine them as forks, where 2 of the prongs are our cards, and we try to match the other 3 with the board.
SC - possible str8s with both cards(underlined are NOT nuts)
---------------------------------------------------------------
48 - 45678
58 - 56789 45678
68 - 6789T 56789 45678
78 - 789TJ 6789T 56789 45678
As you can see, the closer the cards are, the more ways to make str8s. With no gap connectors, 4 ways(3 the nuts). With one gap connectors 3 ways(2 the nuts), with 2 gap connectors 2 ways(only one nuts), and with 3 gap "connectors" only one non-nut way.
Of course you can make many more str8 with one of your cards, but they are never the nuts and are very obvious on the board. Unless you have the top card, you can have no confidence at all. Even with a str8 like 9TJQ
K, you cannot be confident since AK beats you, so don't bet the farm with it
.
A final note: As you approach low connectors and high connectors, there are fewer ways to make a str8. Here is the list of connectors that have FEWER chances because they are too low or too high.
34 - 34567 23456 A2345 (1 less str8)
24 - 23456 A2345 (1 less str8)
A4 - A2345 (1 less str8)
23 - 23456 A2345 (2 less str8s)
A3 - A2345 (2 less str8s)
A2 - A2345 (3 less str8s)
JQ - TJQKA 9TJQK 89TJQ (1 less str8)
JK - TJQKA 9TJQK (1 less str8)
JA - TJQKA (1 less str8)
QK - TJQKA 9TJQK (2 less str8s)
QA - TJQKA (2 less str8s)
KA - TJQKA (3 less str8s)
As you can see, the smallest SCs with full potential(max number of str8s possible) are 45, 35, 25 and A5. In other words, if the high number is 4 or less, you don't get full potential. Furthermore, you still get the non-desirable str8s (the low non-nuts).
On the other hand, for the high cards, if the low card is T, you get full str8 potential. If the low card is J or higher, you lose str8 possibilities. However, they are ALWAYS the nut str8 when you make them, so it is not as bad as with low cards. In addition, since people play high cards more than low cards, if someone has top pair or 2 pairs, it is easier to stack them with these str8s.
Str8s are harder to see, but this is why they make more money than flushes, so it is worth it to draw to them(just make sure you have the nut str8).
I hope these notes on str8s are helpful.