What distinguishes laptop memory from desktop memory is the form factor Form factor is a memory’s physical size and its pin arrangement. Most desktop manufacturers use DIMM. A full-size DIMM has 100, 168, 184, or 240 pins and is usually 4.5 to five inches in length. Most laptop manufacturers use SO DIMM (Small Outline DIMM). A SO DIMM has 72, 100, 144, or 200 pins and is smaller — 2.5 to 3 inches. There is also SO RIMM, which is similar to SO DIMM, but like DIMM/RIMM has a different pin count.