Let's make it even more simple with some fake numbers to make it super easy.

$0-$10,000 - taxed at 10%

$10,000-$20,000 - taxed at 20%

$20,000-$30,000 - taxed at 30%

If you make 9 and get a raise to 11, seems like you would now pay 20% on all that! But no.

The first 10 is still taxed at 10%, only the extra $1,000 gets taxed at 20%.

If you go to 31, again, the first 10 is taxed at 10, the next 10 is taxed at 20, and only the extra $1,000 gets taxed at 30%.