The Claisen condensation proceeds between two esters possessing α-hydrogen atoms promoted by a strong base, leading to a β-keto ester. The overview of this reaction is shown in Figure 1. 
Figure 1. Overview of the Claisen condensation.
Typically, the base used is the alkoxide base of the alcohol formed. This prevents side product formation arising from nucleophilic attack or substitution, as the alkoxide is simply regenerated.
The Claisen condensation is effectively ester analogue of the aldolcondensation. If multiple esters are within the same reaction mixture, a combination of possible products will be obtained. The mechanism for this reaction is illustrated in Figure 2.
Figure 2. Mechanism for the Claisen condensation.
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