Crossed aldol condensation is a variation of aldol condensation.
crossed aldol condensation:
Step 1: The hydroxide ion deprotonates the enolizable aldehyde reversibly.
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.