Ram Rank

Can someone please explain to me the difference between Single Rank RAM and Dual Rank RAM, in terms of;



Physical Space taken - will dual with chips on both sides occupy the same space as single or are there situation where this is an issue (SODIMM)?



RAM seen by the OS - eg if I install 8GB of Single Rank, the OS sees 8GB, if I install 8GB of Dual Rank RAM, will the system see 8 or 16 GB?



Speed, I understand that dual is almost twice as slow.



Any other advantages or disadvantages of each.

Cost - i noticed Dual is about 1.5 times the cost of Single Rank - why is this worth the extra cost if I am taking a performance hit?



Thanks.

 

Here's an excellent article.
https://www.hardwaretimes.com/single-rank-vs-dual-rank-memory-which-one-is-better/