Today I received an email which included the line, “It doesn’t matter who’s responsibility it is…”

Written by a professional who should know better, it was ironic that it was me, and not them, doing a massive facepalm.

This incorrect use of the homophone “who’s” instead of “whose” is a common error, but that doesn’t make it excusable.

The apostrophe in “who’s” signals that it is a contraction— a shortening of two words into one, so that “who is” becomes “who’s”. Alternatively, it can also be a contraction of “who has”. You can tell which one it is by determining if the sentence is in past or present tense,

Examples:
That’s the boy who’s a really good actor.
Who’s in charge around here?
Who’s been eating my porridge?

‘Whose’ is a pronoun of ownership. 

Examples:
This is the farmer whose cows ate all my corn. 
Whose car is that? 

Once you know the difference, it’s fairly straightforward. That means there is absolutely no excuse for getting them wrong, even if they do sound the same when spoken.

Fun fact: “it’s” and “its” work exactly the same way.