APOSTROPHES

Where is the apostrophe placed in a contraction?

The apostrophe in a contraction replaces letters or words that have been omitted:

It's the best of both worlds.  (It is the best of both worlds.)

They won't cooperate.  (They will not cooperate.)

They're determining who's the winner.  (They are determining
who is the winner.)

How is the possessive of a word formed that ends in s?

The apostrophe follows the s when the possessive word, whether singular or plural, ends in s:

The activists' agendas were strikingly similar.

Technicians' and secretaries' salaries are listed as direct costs.

Mr. Jones' report was deemed unacceptable.

The albatross' wings are long and narrow.

Is an apostrophe used when forming the plural of letters, figures, abbreviations, acronyms, etc.?

Although an apostrophe may precede the s when forming plurals of letters, signs, symbols, figures, etc., the trend is to omit the apostrophe unless it causes confusion to do so:

In this case, there are no if's, and's, or but's.

The case study covered programs which were carried out in
the late 1940s. (or 1940's)

The majority of personnel here at the laboratory have PhDs.
(or PhD's)

A number of GFIs were installed to reduce the potential for
massive electrical shock. (or GFI's)

Responses followed by Xs are simply tallied and then ignored.
(or X's or x's but not  xs)

Most of the grades issued by the professor were A's and I's.
(not As or Is nor as  or is)

Is an apostrophe followed by the letter s used to form the possessive of it and who?

No. The possessive of it is its. It's is the contraction of it was or it is. Similarly, the possessive of who is whose rather than who's. Who's is the contraction for who is or who has.

Its collar is leather with embedded diamonds. Possessive

We're all interested in whether it's (it is) possible. Contraction

The girl whose car was stolen is waiting outside. Possessive

Who's in charge of this activity? Contraction