[Abcdefg]

You mean this:

Section 1. Tax imposed

(a) Married individuals filing joint returns and surviving spouses
There is hereby imposed on the taxable income of -
(1) every married individual (as defined in section 7703) who
makes a single return jointly with his spouse under section 6013,
and
(2) every surviving spouse (as defined in section 2(a)),
a tax determined in accordance with the following table:

[VRWC_minion]

What part of "tax imposed" don't you understand ?

While at it, take a look at reg.1, its more descriptive.