SO-DIMM’s modularity requires more physical space for the memory sockets and for modules to stand up from the motherboard. An increasing number of vendors are dispensing with SO-DIMMs on ultralight designs, such as the MacBook Air. Here, fromiFixit’s mid-2011 teardown of the 13” Air, you can see 4 GB of Hynix memory (outlined in red) soldered directly onto the mainboard. This is great for space savings, but upgrading becomes impossible.