Every memory slot can be thought of as having a "maximum" memory size you can put into it. It’s best not to exceed this maximum module size to ensure that the memory you purchase will work as intended. In some cases the maximum size of memory per slot in your system will be specified for you. In other cases the total amount of memory that your computer can support will be specified and you can divide that amount across your slots. For example, if your motherboard can support up to 4GB of memory and you have two slots, the maximum memory size you can use is 2GB in each slot (2GB in each slot X 2 slots = 4GB).