System networks

**Figure 1.3:** A basic system diagram
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A system consists of an entry condition and a set of output features. There is no upper limit to the number of output features, but most systems have only two. The basic form of a system is shown in Figure 1.3. This system has the system name ``MOOD TYPE'', a single entry condition ``clause'', and two alternative output features ``indicative'' and ``imperative''. The system represents the paradigmatic choice between these features, for major clauses in English.

An output feature of one system may be the entry condition for another system. In this way, systems are linked together into a system network. A fragment of a system network for the English mood system is shown in Figure 1.4. This figure contains three separate systems: MOOD TYPE, INDICATIVE TYPE and INTERROGATIVE TYPE. The output feature ``indicative'' of the MOOD TYPE system is also the entry condition of the INDICATIVE TYPE system, so that there is a dependency between these two systems: the INDICATIVE TYPE system is only entered if ``indicative'' is chosen as the output feature of the MOOD TYPE system.

**Figure 1.4:** A fragment of a system network diagram
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In SFG, the dependencies between systems in the network also has a more general significance. The more fundamental systems occur towards the left of the network, and the more ``delicate'' systems occur towards the right of the network. This left-to-right dimension of the network is called the scale of delicacy. [Mellish 1988] discusses this aspect of system networks in terms of incremental description refinement.

More than one system may share the same entry condition. In this case, the systems are entered in parallel from the entry condition. In the system network diagram a left curly bracket { is drawn from the entry condition, spanning all the simultaneously entered systems. An example is shown in Figure 1.5.

**Figure 1.5:** Simultaneous systems
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A system may have more than one entry condition. If two or more entry conditions are conjointly necessary for entry to the system, they are linked to it by a right curly bracket }. If any one of two or more entry conditions is disjointly sufficient for entry to the system, they are linked to it by a right square bracket ].

Systems represent paradigmatic choices not only between grammatical alternatives, but also between lexical alternatives. The lexicon is considered as a thesaurus. There is no clear division between grammar and lexicon, and Halliday uses the term lexicogrammar to include both. In general, grammatical choices occur towards the left of the network, and lexical choices occur towards the right of the network. This has given rise to the expression lexis as most delicate grammar.

The explicit description of paradigmatic choices is what distinguishes SFG from other approaches to grammar. Halliday described the choices which are available in a language as the ``meaning potential'' of that language. The systems show meanings, which are realized in the structures of the language as wordings (see Section 1.2.2).