Why the new 12000 dollar standard deduction for single, and 24000 for joint in the new tax plan is

[Ms. Toad]

50. You are the one getting confused.

39a is an additional deduction, taken on top of either the standard or itemized (line 40).

From IRS Pub 551:

Additional Standard Deduction - You're allowed an additional deduction if you're age 65 or older at the end of the tax year. You're considered to be 65 on the day before your 65th birthday. You're allowed an additional deduction for blindness if you're blind on the last day of the tax year. For example, a single taxpayer who is age 65 and blind would be entitled to a basic standard deduction and an additional standard deduction equal to the sum of the additional amounts for both age and blindness.

https://www.irs.gov/taxtopics/tc551

The Senate version (adopted during reconciliation) left the deduction for elderly/blind "unchanged."

It is (and always {recently, at least} was) part of the standard deduction (not an additional deduction on top of itemized deductions before):

From Pub 501 for the 2016 tax year:

Higher Standard Deduction for Age (65 or Older)
If you are age 65 or older on the last day of the year and don't itemize deductions, you are entitled to a higher standard deduction. You are considered 65 on the day before your 65th birthday. Therefore, you can take a higher standard deduction for 2016 if you were born before January 2, 1952.

Use Table 7 to figure the standard deduction amount.

Death of taxpayer. If you are preparing a return for someone who died in 2016, consider the taxpayer to be 65 or older at the end of 2016 only if he or she was 65 or older at the time of death. Even if the taxpayer was born before January 2, 1952, he or she isn't considered 65 or older at the end of 2016 unless he or she was 65 or older at the time of death.
A person is considered to reach age 65 on the day before his or her 65th birthday.



Higher Standard Deduction for Blindness
If you are blind on the last day of the year and you don't itemize deductions, you are entitled to a higher standard deduction.

Not totally blind. If you aren't totally blind, you must get a certified statement from an eye doctor (ophthalmologist or optometrist) that:
You can't see better than 20/200 in the better eye with glasses or contact lenses, or
Your field of vision is 20 degrees or less.
If your eye condition isn't likely to improve beyond these limits, the statement should include this fact. Keep the statement in your records.

If your vision can be corrected beyond these limits only by contact lenses that you can wear only briefly because of pain, infection, or ulcers, you can take the higher standard deduction for blindness if you otherwise qualify.

https://www.irs.gov/publications/p501#en_US_2016_publink1000221057

In other words - nothing has changed. It has been that at least since 1990 (as long as I've been playing with taxes). *I've seen references to a switch from an exemption to a deduction as part of the mid-80s Reagan tax reform, but can't track that change down precisely). But as to changes imposed by Trump's tax law - before the new law you could only take it if you were NOT itemizing. The new tax bill doesn't change that.