2.1.4　Local coherence

Compared to global coherence，the term local coherence has a much more agreed connotation in the linguistic circle，as the review of the distinction between global coherence and local coherence in Sub-section 2.1.3.1 has shown.It concerns the interrelatedness between adjacent units like sentences，in discourse segments.In this section the review will be focused on the major existing theories on local coherence.

The term local coherence is，like global coherence，also usually used to refer to relations inside the text in purely linguistic sense.Specifically，it refers to the semantic and logical inter-sentential relations or relations between units somewhat larger than sentences，such as clause complex.For example，van Dijk and Kintsch（1983：150）claim，“under local coherence we understand a property of discourse which is defined in terms of semantic relationships between the successive sentences of the discourse.”Samet and Schank（1984）hold that local coherence relations include inter-sentential relations such as cause-consequence，temporal succession，reason-result.Encyclopedia of Linguistics defines local coherence as“the interrelatedness between adjacent discourse segments”（Louwerse&Graesser，2005：217）.

Local coherence need not be signaled;actually，as Rhetorical Structure Theory suggests，some types of rhetorical relations have no corresponding conjunctive signals（Hu and Ye，2010：178）.

There has been a large body of work on the inter-sentential relations in linguistic texts.From the perspective of text analysts，the two most influential theories are Rhetorical Structure Theory and Halliday's framework of clause complexes.

2.1.4.1　Rhetorical Structure Theory

As an influential approach to text coherence and organization of written discourses，Rhetorical Structure Theory addresses text organization by means of relations that hold between parts of a text.It explains coherence by postulating a hierarchical，connected textual structure in which every part of a text has a role to play，a function to fulfill，with respect to the other parts of the text.The notion of text coherence through text relations is widely accepted;the relations have also been called coherence relations，discourse relations，or conjunctive relations in the literature（Taboada，2006）.Text coherence is attributed principally to the presence of these relations（Hu and Ye，2010：177）.Relations in RST are defined in terms of the four fields：

（1）Constraints on the nucleus;

（2）Constraints on the satellite;

（3）Constraints on the combination of nucleus and satellite;

（4）Effect（achieved on the text receiver）.

In terms of the four fields，a set of relations are defined on the basis of functional and semantic criteria and the list is being continuously expanded with the encounter of new text types and examples.Generally the relations fall into two groups：presentational and subject matter related.Presentational relations，such as Antithesis，Background，Concession，Enablement，Evidence，Justification，Motivation，Preparation，Restatement，Summary，are those whose intended effect is to increase some inclination in the reader，such as a desire to act，or to heighten the degree of positive regard for，belief in，or acceptance of，the nucleus.Subject matter relations，such as Cause，Circumstance，Condition，Elaboration，Evaluation，Interpretation，Means，Purpose，Result，Solutionhood，are those whose intended effect is that the reader recognizes the relation in question.

An analysis of RST is usually done by reading the text and constructing a diagram in which all relations are explicitly identified.Every relation is defined in terms of intentions that lead authors to use that particular relation.Thus，an RST diagram provides a view of some of the author's purposes or intentions for including each part.

The strengths of RST include its emphasis on a functional conception of relations，the carefully presented set of relation definitions and the simply stated structural theory（Hu and Ye，2010：179）.The weakness of RST analysis is that it is inherently subjective，being based on a reader's understanding of texts.When analysts study and diagram texts，they use their knowledge of the culture，situations，and language that the texts represent（Taboada，2006）.

In all，RST provides a very useful framework for the analysis of text coherence.However，coherence in discourse is achieved by different means.Coherence relations—relations that hold together different parts of the discourse—are only partly responsible for the perceived coherence of a text（Taboada，2006）.Therefore，in order to fully account for the construction of coherence in discourse，rhetorical relations are not adequate，and other means of coherence construction have to be considered.

2.1.4.2　Halliday's“clause complexes”

Halliday's“clause complexes”is another theory that has been widely applied in the description of local coherence in discourses.Halliday（1994）classifies the relations between sentences，or“clause complexes”in their terminology，in two dimensions：（i）the type of interdependency，or Taxis;（ii）the logico-semantic relation.In the first dimension，two general categories，hypotaxis and parataxis are described.The former is the relation between a dependent element and its dominant.The latter is the relation between two elements of equal status，one initiating and the other continuing.Along the second dimension，he discusses the various logico-semantic relations which might hold between two clauses in a clause nexus.Two general types are identified：expansion and projection.By expansion，the secondary clause expands the primary clause，by elaborating，extending or enhancing it.By projection，the secondary clause is projected through the primary clause，which instates it as a locution or an idea.Under each of these major categories，there are finer classifications which will not be elaborated here.His main framework is presented in Table 2.4 below：

Table 2.4　Basic types of clause complex（Halliday and Matthiessen，2004/2008：380）

The strength of Halliday's“clause complexes”is that it provides a systematic classification and can be easily operated and applied in the analysis of texts.It has been proved by its popularity in the field of text analysis.However，the weakness is that it emphasizes more on the grammatical and textual relations between clauses than on the real logico-semantic relations some of which are frequently encountered in analysis.The reason for this weakness is Halliday's confusion between semantic relations and rhetorical relations in the criteria of classification（Cheng，2005）.The major suggestion of Cheng（2005）is to replace the logico-semantic relations by two parallel categories：semantic relations and rhetorical relations.Semantic relations refer to the relations between the events represented by the clauses，such as cause-effect and condition relations，and they are objective.In contrast，rhetorical relations refer to those relations between the elements in the text.