Line 12a, Form 1040 Where does this calculation come from?

[Mpharis27]

The IRS has contact me to send backup information for the amount on line 12a of my 1040 form for 2019 taxes

[Mike9241]

there are multiple possibilities

here's a link to 1040 instructions for line 12a - look on page 31. (yes the instructions are for 2020 return but they are the same as 2019)

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf 

[Mpharis27]

I found the 2019 tax instructions

www.irs.gov/pub/irs-prior/i1040gi--2019.pdf

Line 12a of 1040 is calculated from the tax instruction table based on adjusted income