P.L. 115-97 replaced the graduated corporate tax structure with a flat 21% corporate tax rate, effective for tax years beginning after December 31, 2017. Therefore, the maximum tax rate for corporations has decreased from 35% to 21%. However, under section 15, corporations and section 1446 partnerships with fiscal tax years beginning before January 1, 2018, and ending after December 31, 2017, figure their tax by blending the corporate rates in effect before January 1, 2018, with the rate in effect after December 31, 2017. For more information see the Instructions for Form 1120, available at IRS.gov/Form1120, and other impacted forms and instructions posted on or after April 13, 2018, and available at IRS.gov/DownloadForms. More information is available in Notice 2018-38.
Below is a list of impacted forms and instructions.