Residues present in the interface of interacting partners are solely responsible for communication between the partners. There are several studies on PPI, which state higher conservation of interfacial residues than remaining protein surface. It is also known that interfaces of permanent protein complexes have a lower evolution rate than transient interactions, which allows better co-evolution with its interacting protein partner. Similarly, in this case, we ought to look into the conservation at the domain interfaces of different DDI types in a protein chain. Due to the requirement of ConSurf-DB to have at least a certain number of homologues to follow the evolutionary rate, some of the proteins in our dataset could not be retrieved from the database. Hence, we analyzed only 101 and 147 permanent and transient domain pairs in proteins, respectively. We computed the absolute number of interfacial residues which are conserved in domain pairs and observed that permanent domain pairs have a little wider distribution of the number of conserved interfacial residues than transient domain pairs, which could be due to a large number of interfacial residues arising from larger interfaces. But the number of such residues in both domain types is not significantly different to account for any dissimilarity (with a Mann-Whitney p-value: 0.1341 and two sample KS test: 0.1882). Next, we also computed the normalized number of conserved interfacial residues, that is, the number of conserved interface residues per total number of interface residues in a domain pair of a protein, and we observed near similar distribution of such residues with respect to their interface residues with near identical mean and median (Figure 5). These observations suggest that both permanent and transient domain pairs have similar proportions of conserved interfacial residues. Unlike PPIs, this similarity in maintaining the conservation at the interface could be due to the fact that the interacting partners, here domains, are referred as semi-independent and evolutionary units which are thought to be conserved. Like permanent PPI, residues in the permanent domains might be under evolutionary pressure to co-evolve with partner domains. On the contrary, two transient domains might be harboring functional sites at their interface and hence the obligation to preserve the interfacial residues. Each domain interaction type has its compulsion to maintain the interface geometry, resulting in similar preferences to conserve the domain interfaces in the two-domain protein.