ThinkPad X301 Service and Troubleshooting Guide - Lenovo

7. With the notched end of the SO-DIMM toward the contact edge side of
the socket, firmly insert the SO-DIMM into the socket at an angle of
about 20 degrees ▌1▐; then pivot it downward until it snaps into place
▌2▐.
1
1
2
2
Note: If you install a memory module in only one of the two memory
slots, install it in the slot on which “SLOT-0” is printed.
Chapter 5. Upgrading and replacing devices 43