Wanted: Gap Concept Explanation

Just finished reading HOH-1. I really liked this book and as a newbie to poker I got a lot out of it. That said, there is something in the book that is repeated often but not explained. Harrington often refers to the "Gap Concept" when playing. I think it has to do with your starting hand requirements going up when entering a raised pot, but I don't understand what the rule(s) actually is. Can someone explain the Gap Concept, please?
Thanks,
Mark

The rule is that it takes a stronger hand to call a bet or raise than it takes to raise yourself if its checked/folded/limped to you.

Preflop you might want to raise AJs if its folded to you in late position, but if there's a raise before you its normally an easy fold.

The same concept applies post flop too.

The size of the gap depends on the playing style of your opponents. Against tight players the gap is wider and against loose players the gap is narrower.

A way i like thinking about this, is imagine you and your opponent have the exact same hand. Say you both have AK. In what situations would one of you be able to make the other one fold?

An example would be, say one of you raises preflop to 4xbb with your AK. The other one calls with his AK. The flop is all rags, and the preflop raiser bets 8xBB on the flop. AK #2's hand is not good enough to call here in almost all circumstances, while AK #1's hand is good enough to bet with a lot of the time.

Here's an explanation of the gap concept for preflop play. A player raises UTG. For simplicity, we will say his preflop raising range is JJ-AA and nothing else. If you call with the same range, you are making a big mistake. If you call with JJ, 3 out of 4 times he has you dominated with a bigger pp, and 1 out of 4 times you have the same hand. This is clearly a horrible situation for you. So you need to tighten up and call with a much smaller range than the initial raiser's range (and usually you don't want to call a raiser preflop ... you reraise or fold). In this simplified case, you would reraise KK and AA and fold any other hand. As a rough guideline, you want to play the top half of the raiser's range. This gives you a 75% chance of having a better hand. To see why, assume your opponent raises with the best 10% of hands. If you now play only the top 5% of hands, you beat the bottom half of his range always, and you are 50-50 against the top half. 50%+50%*50%=75%. By the same reasoning, if you play hands in the bottom half of the 10% range, your opponent has you beat 75% of the time. This is why you must have a much stronger hand than the bottom of the raiser's range to play ... you will usually be dominated by a much stronger hand.

Originally Posted by Greedo017

A way i like thinking about this, is imagine you and your opponent have the exact same hand. Say you both have AK. In what situations would one of you be able to make the other one fold?

An example would be, say one of you raises preflop to 4xbb with your AK. The other one calls with his AK. The flop is all rags, and the preflop raiser bets 8xBB on the flop. AK #2's hand is not good enough to call here in almost all circumstances, while AK #1's hand is good enough to bet with a lot of the time.

The same is also true for the opposite. AK #1's hand is probably not good enough to raise in the first place, but if checked to AK #2, he has an easy opportunity to raise making it tough for AK #1 to call since he would be first to act again on the turn and would probably lose more money. A re-raise might get AK #2 out, but a call still puts you in a tough position.

Position, Position, Position!!!

You get a range for a players hands based on their position, you only call a raise or even a limp preflop with a better hand. The gap is the difference in position. If you're on the button and would normally call with KQ, but a guy limped UTG who would only limp with AK and pairs, you probably get out of the way, despite your position.