Crossed aldol reaction is a variation of aldol reaction.

aldol reaction:

crossed aldol reaction:

eg:

Mechanism:
Step 1:  The hydroxide ion deprotanates 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.

see also crossed aldol condensation