PC3-12800/1600 MHz DDR3 SO-DIMM - 204 pin
For 2012-15 MacBook Pro 13in. and 15in.
all models have 2 slots, each can take an 8GB module.
For 2012 through Late 2015 iMac 27 in
27 in. iMac has 4 slots, each can take an 8 GB module