Retrievability of memory is a variable of long-term memory that corresponds with the memory clock. Internally, retrievability may determine the increase in memory stability via re-activation (see: spacing effect). Outwardly, memory clock will show as a decline in the probability of recall. The name comes from the fact that the longer the time since the last review, the less the chance of retrieving the memory. We can map retrievability with the help of the forgetting curve.