The term "gap concept," first introduced by renowned poker author David Sklansky, refers to the idea that a player needs a better hand to call a raise than to open the pot themselves.
For example: if it is folded to you in middle position it may be correct to open-raise AJ; however, if you're facing a raise from a tight player under the gun, AJ's value shrinks dramatically.
This is clearly because an early-position player, when they raise, feels that their hand is already better than the rest of the table.
After all, their raise has to make it through the entire table and each player left to act could potentially hold a better hand.
If, however, he raises from late position, they're really only saying "my hand figures to be best against everybody else." Thus an early-position raise holds a lot more weight than a late-position raise because it needs to beat more players.
How the Gap Concept Works in Poker
Which is why when you are facing a raise, you should tighten up substantially. You must have a hand that is stronger than what you would have required to open the pot for a raise yourself.
This is to make up for two things: the lack of initiative you have from calling, and the fact that your opponent is telling you that he thinks his hand is strong enough to beat the field.
If your hand isn't stronger than your normal raising range from that position, you risk "being dominated."
Being dominated means you and your opponent share your biggest card but he has your kicker beaten. For example your opponent has AK and you have AQ - a recipe for disaster.
Playing a dominated hand can be extremely problematic, and this is why the "gap concept" was born. Those times that you do hit your ace you are going to end up spending a lot of money to find out you are second-best at showdown. The gap concept can help save you from many of those.
What the actual gap is is by no means set in stone. Ultimately, the gap can shrink or grow depending on the player opening the pot and the situation.
If your opponent plays very nitty, then the gap may be very wide. If, however, your opponent is a loose-aggressive player, your gap might be very small or even nonexistent.
Many players feel that "the gap concept" is an antiquated piece of poker strategy, with no real place in today's aggressive games. This is not true.
Although the rule itself has become a bit dated, the logic behind it remains the same. When you call, you want to have a hand that figures to be best now - or has good equity against your opponent's hand.
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