Crossed Aldol

The Crossed Aldol (sometimes called the Claisen-Schmidt reaction or mixed aldol reaction) results when two different aldehydes are present.

As you can imagine it's not very synthetically useful since 4 possible products are formed and the products are difficult to separate since they are all very similar.

Crossed aldol reactions are useful however when one of the aldehyde/ketones can not form an enolate.  For example, the reaction of pivaldehyde with acetaldehyde in the presence of OH- will undergo a directed crossed aldol.  Furthermore to prevent self-condensation of the acetaldehyde, a solution of pivaldehyde is slowly added to a mixture of OH- and acetaldehyde.  Alternatively you can use a powerful base such as LDA and convert all the acetaldehyde to enolate and then add the pivaldehyde slowly.