In the pre-2018 W-4 Personal Allowance Worksheet, you get one allowance for each exemption, i.e. person on your tax return -- you get 1 for yourself, 1 for your spouse, 1 for each dependent. (This makes sense since before 2018, you get a $4050 deduction for each exemption.)
In 2018 and later, you don't get a deduction for exemptions anymore. Yet the 2018 withholding tables "are designed to work with the Forms W-4 that workers have already filed with their employers to claim withholding allowances" (i.e. pre-2018 W-4 forms). I am wondering how that is possible since there is no way for the employer to know how many of the allowances were due to exemptions, and thus don't lead to deductions anymore, and how many of the allowances were due to other things, which still correspond to deductions (the employer only gets the stub with the number of allowances, but not the worksheet to calculate it).
I am guessing that maybe the tables were made assuming a certain "common" number of exemptions (perhaps 2 or 3), since they say "For people with simpler tax situations, the new tables are designed to produce the correct amount of tax withholding." But if you have many dependents, and thus many of the allowances were due to exemptions, does that lead to under-withholding in 2018, since the tables were made assuming fewer of your allowances are exemptions and more of your allowances correspond to actual deductions, when in reality many of them don't correspond to deductions?