A. A brief introduction to register theory
(see also SGSWE pp. 4-5)

Language varies according to the situation in which it is used, and these varieties of language can be referred to as registers. If we examine a text we can make guesses about the situation; on the other hand, if we are in a particular situation we make certain linguistic choices based on that situation. In other words, the language we use needs to be appropriate to the situation in which we use it.

Field, tenor, mode
The linguist Michael Halliday divides these variables into three categories:
Field, Tenor and Mode.
These three variables combine to form the register of the text. Here is an outline of the three categories.
Field: what language is being used to talk about
The field includes: the topic; the interactants.
The topic of discourse can be: specialised/technical (e.g. talking about the environment etc.); everyday (e.g. talking about shopping etc.)
The interactants may have: specialised knowledge of the field (e.g. a scientist writing for an article for an academic journal); common knowledge of the field (e.g. the readers of a newspaper article).
The field determines the use of specialised language, for example whether technical terms or everyday terms are used. It is of course always important for a writer to think about his/her audience (who he/she is writing for), which may be specialist or wide-public (see SGSWE p.4). Look for example at these two examples, one from an academic journal and the other from a daily newspaper.
The net ecosystem exchange of carbon dioxide was measured by eddy covariance methods for 3 years in two oldgrowth forest sites near Santarém, Brazil. Carbon was lost in the wet season and gained in the dry season, which was opposite to the seasonal cycles of both tree growth and model predictions. The 3-year average carbon loss was
1.3 (confidence interval: 0.0 to 2.0) megagrams of carbon per hectare per year. Biometric observations confirmed the net loss but imply that it is a transient effect of recent disturbance superimposed on long-term balance.
Failure to take action to combat climate change will cause environmental catastrophe and cost the global economy
$20 trillion (£10.8 trillion) a year by the end of the century, the pressure group Friends of the Earth says today. In a report based on research from more than 100 scientific and economic papers, the group says allowing global warming to continue unchecked will mean a temperature rise of 4C by 2100, causing economic damage worth up to
8% of global GDP. (The Guardian 13.10.2006)

Tenor of discourse: the role relationships between the interactants
The relationships between the interactants varies according to: status (ranging from unequal as in the case of a boss/ employee to equal as with friends); affective involvement (ranging from high as with friends/family members to low as with business clients); contact (ranging from frequent to occasional).

The relationship between interactants influences the formality of the language used (see section
“Language differences between registers”). Look for example at these two examples of emails, one from a seminar organiser to a participant (unequal status; low affective involvement; occasional contact) and the other from a family member (equal status; high affective involvement; frequent contact).
We are very pleased to hear that you have been appointed to represent CercleS at the 7th ELP Seminar in Vilnius,
Lithuania.
I would be very grateful if you could let me have your full postal address so that we can send you a signed invitation. In the meantime, please find attached the draft programme and other documents concerning the seminar.
HIIII!! thanks sooo much for sponsoring me. im very very grateful. how are things? so you can probably tell from the late reply of this email that i havent really got myself organised, though i have just sent off my accommodation application