CSS :nth-last-child Selector

/* Sets the color to red if the p element is its parents second to last child */
p:nth-last-child(2) {
  background: red;
}

Meaning

The :nth-last-child() selector selects the element that is the nth from last child of its parent.

The :nth-last-child() pseudo-class is specified with a single argument, which represents the pattern for matching elements, counting from the end.

Version: CSS3

:nth-last-child(n|odd|even|An+B) {
  css declarations;
}

The numbers in the table specify the first browser version that fully supports the property.

The following table describes the values of this selector parameter.

Value	Description
n	Specifies the number to selects the element that is the nth from last child of its parent.
odd	Defines elements whose numeric position in a series of siblings is odd: 1, 3, 5, etc.
even	Defines elements whose numeric position in a series of siblings is even: 2, 4, 6, etc.
An+B	Defines elements in a list whose indices match those found in a custom pattern of numbers, defined by An+B, where: A is an integer step size B is an integer offset n is all nonnegative integers, starting from 0.

tr:nth-last-child(odd) or tr:nth-last-child(2n+1): Represents the odd rows of an HTML table: 1, 3, 5, etc., counting from the end.
tr:nth-last-child(even) or tr:nth-last-child(2n): Represents the even rows of an HTML table: 2, 4, 6, etc., counting from the end.
:nth-last-child(7): Represents the seventh element, counting from the end.
:nth-last-child(5n): Represents elements 5, 10, 15, etc., counting from the end.
:nth-last-child(3n+4): Represents elements 4, 7, 10, 13, etc., counting from the end.
:nth-last-child(-n+3): Represents the last three elements among a group of siblings.
p:nth-last-child(n) or p:nth-last-child(n+1): Represents every <p> element among a group of siblings. This is the same as a simple p selector. (Since n starts at zero, while the last element begins at one, n and n+1 will both select the same elements.)
p:nth-last-child(1) or p:nth-last-child(0n+1): Represents every <p> that is the first element among a group of siblings, counting from the end. This is the same as the :last-child selector.