Contrast with Fluid Intelligence
Where fluid intelligence finds a rule in a problem met for the first time, crystallized intelligence depends on how well stocked your store of knowledge is and how readily you can reach into it. The two are not independent: understanding gained through fluid work accumulates and becomes part of the crystallized store. Their relationship with age is the opposite of each other - fluid intelligence peaks relatively early in adulthood and then declines gradually, while crystallized intelligence can keep growing for a long time as experience accumulates. That overall intellectual functioning is often maintained at a high level well into middle age and beyond is explained partly by these two trajectories offsetting one another.
Its Place in Measurement
In standardized intelligence tests, subtests probing vocabulary and general knowledge capture the crystallized side. Because these items are strongly affected by schooling and native language, they are ill suited to comparing populations that differ in culture or language. Conversely they are useful for gauging how far learning has progressed within a group that shares an educational background. Two people with the same composite score can have completely different profiles depending on whether their strength lies on the fluid or the crystallized side, so the character of someone's ability cannot be inferred from the number alone.