I found this to be a confusing subject so I wanted to share what I have learned. I finally figured it out from this website.
vi editor FAQ (Frequently Asked Question List), Part 1/2
You MUST include the single double quotation in this. Obviously <letter> with any letter you feel like.
"<letter>yy
Another example with deleting text.
In this example this deletes a single letter and puts it in the x register. You do not need the brackets here. They just used them to make the single double mark more obvious.
["x]x