There are worksheets attached to Form W-4. These include a Personal Allowances Worksheet, a Deductions and Adjustments Worksheet, and a Two-Earner/Two-Job Worksheet. The worksheets are to help you determine the number of withholding allowances to claim, and are for your personal records. You do not necessarily need to claim the number of withholding allowances that result from completing these worksheets. And the figures you use to complete these worksheets do not obligate you with regard to the number of exemptions, deductions, or credits you can actually claim when you file your annual income tax return.
The worksheets are intended as a guide, and will generally result in the appropriate amount of tax being withheld from your pay if you complete the worksheets correctly and you can foresee and estimate the items that will affect your tax situation for the year in a reasonably accurate manner. You do not have to complete all three worksheets. You should generally complete the Personal Allowances Worksheet, and should complete the other two worksheets if they apply in your case.
If You Have More Than One Job
You should complete only one set of worksheets for Form W-4 purposes, even if you have more than one job. You cannot claim the same allowances with more than one employer at the same time. But you can then divide your total number of withholding allowances between the W-4 Forms you have to file with each employer in any way you want. You could claim all your withholding allowances on one Form W-4 and none on the other(s), or divide them up any way you choose.
If You Are Married
If you are married, you and your spouse should complete one set of worksheets for both of you if you plan to file jointly. One spouse cannot claim a withholding allowance claimed by the other spouse. But you can divide up your total withholding allowances between you in any way you choose. If you plan to file separately, you should each complete your own set of worksheets.