Can I use laptop RAM in a desktop?

Solution 1:

They are physically different sizes and cannot be used interchangeably on that fact alone. DDR2 SO-DIMMS (commonly found in laptops) have 200 pins and regular DDR2 DIMM's have 240 pins.

Solution 2:

As it seems the question is still getting views and being used to close new questions as duplicates I'll add the current answer.

There exist adapters, both for DDR3 and DDR4 that allow using SO-DIMM modules in a DIMM socket, here is an example from Aliexpress.

They have been tested by multiple people to be working, for example Linus Tech Tips.

There is a difference in pin count between the connectors but the added pins ade not connected, optional or duplicates.