Clitic climbing is a phenomenon first identified in Romance languages in which a pronominal object of an embedded infinitive appears attached to the matrix verb. Pronominal objects in Romance languages are typically expressed as clitics. Hence the name clitic climbing. The following Italian example illustrates the phenomenon. The object pronoun, lo, a clitic, is attached to the infinitive in the embedded or subordinate clause in (1a). In (1b), the clitic has "climbed" to the main or matrix clause and is attached to the matrix verb. There is no discernible difference in meaning between the two forms.
(1) a. Gianni vuole comprar=lo. Gianni wants to.buy=it "Gianni wants to buy it." b. Gianni lo=vuole comprare. Gianni it=wants to.buy "Gianni wants to buy it."
Clitic climbing is found in almost all Romance languages. It is notably absent in French.
Other language families
(2) a. S‹in›ubuk.an ni Juan =ng dalaw.in=siya. ‹PFV›try.PSV GEN Juan COMP visit.PSV=3SG.NOM "Juan tried to visit her." b. S‹in›ubuk.an=siya ni Juan =ng dalaw.in. ‹PFV›try.PSV=3SG.NOM GEN Juan COMP visit.PSV "Juan tried to visit her."
(3) a. S‹in›abi.∅ ni Juan na d‹in›alaw.∅=siya ni Pedro. ‹PFV›say.PSV GEN Juan COMP ‹PFV›visit.PSV=3SG.NOM GEN Pedro "Juan said that Pedro visited her." (or "Juan said that she was visited by Pedro.") b. * S‹in›abi.∅=siya ni Juan na d‹in›alaw.∅ ni Pedro. ‹PFV›say.PSV=3SG.NOM GEN Juan COMP ‹PFV›visit.PSV GEN Pedro "Juan said that Pedro visited her." (or "Juan said that she was visited by Pedro.")
So, the sentence in (2b) is grammatical because the embedded verb, dalawin "to be visited", is not marked for any aspect, whereas the sentence in (3b) is ungrammatical because clitic climbing has occurred out of the embedded clause in which the verb, dinalaw "was visited", is marked for the perfective aspect.
Rizzi, Luigi. 1978. A Restructuring Rule in Italian Syntax. In Recent Transformational Studies in European Languages, ed. Samuel J. Keyser, 113-158. Cambridge, MA: MIT Press.