In his book "The Theory Of Poker", David Sklansky states that in order to call an early open/raise you should hold at least the same or a tighter range than the open/raiser opponent.
Sklansky called this the "Gap Concept". But as the first round of bets go ahead, players are faced with better pot odds that can lead a villain to call with a wider range of starting hands.
As an example look at this scenario:
The blinds are 0.5xbb and 1xbb, now the pot is 1.5xbb.
The first to open is Alice with 3xbb, now the pot is 4.5xbb.
If Bob would call needs at least an equity of 3/(4.5+3)=0.4.
If the Alice range is R1, Bob needs a range R2 such that R1 vs R2 is at least 60% vs 40% in orther to achieve that equity.
Now the action is on Charlie who needs to commit 3xbb to gain 7.5xbb. So he needs a 0.28 equity against the other players. In order to achieve that equity, Charlie needs a distribution R3 such that R2+R1 vs R3 is 70% vs 30%.
How can we unify these two points of view? Or in other words, where does the gap concept come from?