1.2.3　Discourse coherence as a holistic，multi-layered framework

There has been a large body of research on discourse coherence in linguistic sense which mainly focuses on three major areas：（i）cohesive devices between sentences，such as anaphors;（ii）the semantic and logical relations between sentences，like the Rhetorical Structure Theory;and（iii）the more global factors of discourse coherence，such as discourse topic，macro-structure，story-line，etc.

It is held by many linguists and also in this book that these research concerns belong to different levels of discourse.The cohesive devices operate on the surface level，the inter-sentential semantic relations on the local level，whereas the discourse topic，macrostructure and story-line reflect a global facet of coherence.They are termed as surface cohesion，local coherence and global coherence respectively in this book.All the three levels of relations contribute to the construction of discourse coherence，and an adequate analysis of coherence construction in any genres of discourse needs to take all the three levels into consideration.

Therefore，in order to conduct a systematic and thorough analysis of how coherence is constructed in the English learning advertisements as a multimodal genre，this book develops a three-level theoretical and analytical framework of discourse coherence.Moreover，the notions of the three levels are all extended to accommodate the multimodality in semiotic expression in the English learning advertisements.Specifically，global coherence refers to the macro-control on the smaller semiotic units by the higherorder topic of the discourse，local coherence refers to the semantic relations between the verbal and visual elements in discourse sections，and surface cohesion refers to devices in the layout that help signal the coherence relations and connect individual semiotic units into an organized and cohesive representation.

The three-level framework of coherence directly influences the structure of the book.That is，each of the three levels of coherence is given a chapter for the proposal of working definition and principle，and detailed semiotic analysis and discussion of the data.