Crossed aldol condensation is a variation of aldol condensation.
crossed aldol condensation:
Step 1: The hydroxide ion deprotonates the enolizable aldehyde reversibly.
Step 2: Enolate ion 1 preferentially adds to the non-enolizable aldehyde, which has the sterically less hindered and, therefore, more accessible carbonyl carbon.
Step 3: Alkoxide ion 2 is protonated by water.
Step 4: Aldol 3 is an enolizable aldehyde. A small amount of it is converted to the corresponding enolate ion (4) by the hydroxide ion.
Step 5: Enolate ion 4 loses a hydroxide ion.
Step 1 through 3 are a crossed aldol reaction, steps 4 and 5 a 1,2-elimination via E1cB mechanism. Thus, crossed aldol condensation is crossed aldol reaction followed by 1,2-elimination.