Given a suitable extension $ F'/F $ of algebraic function fields over a finite field $ \mathbb{F}_q $, we introduce the conorm code $ \operatorname{Con}_{F'/F}( \mathcal{C}) $ defined over $ F' $ which is constructed from an algebraic geometry code $ \mathcal{C} $ defined over $ F $. We study the parameters of $ \operatorname{Con}_{F'/F}( \mathcal{C}) $ in terms of the parameters of $ \mathcal{C} $, the ramification behavior of the places used to define $ \mathcal{C} $ and the genus of $ F $. In the case of unramified extensions of function fields we prove that $ \operatorname{Con}_{F'/F}( \mathcal{C})^\perp = \operatorname{Con}_{F'/F}( \mathcal{C}^\perp) $ when the degree of the extension is coprime to the characteristic of $ \mathbb{F}_q $. We also study the conorm of cyclic algebraic-geometry codes and we show that some repetition codes, Hermitian codes and all Reed-Solomon codes can be represented as conorm codes.
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