MacBook Pro memory upgrade

I have a late 2011 MacBook Pro 13.3 in. with 4 GB RAM. I want to upgrade to 8 GB, and all the websites I have looked at to buy the upgrade seem to send it with 2 x 4 GB chips, so I am wondering if I can buy 1 more 4 GB chip and just put that in and have 8 GB or if the system doesn't work that way, and even though there are 2 4 GB chips they are somehow connected, so you have to buy it as an 8 GB upgrade?

Get two 4gb pieces.

Adding a single 4GB module would have give you a total of 6GB rather than 8 because they come with 2 2GB modules in them.

Oh, I see! Great thanks a lot!

Most modern computers run either dual channel or triple channel RAM, so you would want to install matched sticks of RAM either 2 or 3 at a time based on the specific system. The Macbook Pros are using dual channel RAM, so getting an 8GB kit (2 x 4GB) would be better than mixing a 2GB and 4GB stick.