There were three purposes for writing a letter in the Greco-Roman world, as in our own. First, one wrote a letter to convey information to someone at a distance. A second reason for writing a letter was to maintain an existing relationship. Third, one wrote a letter in order to respond to previous correspondence. Although some letters were controlled by one or more of these motives primarily, all three of these motives are usually operational within a single letter. We can call these the informational, relational, and dialogical dimensions of a letter. Extrapolating from this single genre on the basis that all SBT communiation is an outgrowth of ancient epistolary discourse, we can claim that all SBT communication has these three dimensions.

If we focus on their informational function, letters can be divided into three major types: accusatory, petitionary, and eulogistic. These, in turn, are best seen as paired. Accusatory letters are paired with apologetic letters: they make up a speech set. Petitionary letters are often paired with advisory letters. Although one might send an unsolicited advisory letter, most advisory letters are responses and thus part of a speech set. A variation of this speech set is the directive letter (one that issues a order) and the response, which may take a number of forms depending on the directive. These pairs make up speech sets. The first letter calls forth a response in the second. The last pair is contrastive rather than responsive: paired with eulogistic writing is vitributive writing. Whereas the motive of the first is to praise, the motive of the second is to blame.

Also associated with the informational dimension of letters is the art of invention. How does one come up with things to say to fill out the message? Here, a study of what was once called the topoi--but now often referred to as the modes--may be in order. Other heuristic procedures or research methods fall naturally under this heading.

As we consider their relational function (maintianing good will), we focus on letters' tone (i.e., their attitude vis a vis the subject and audience), their directness or evasiveness, their strategies for conveying good and bad news, and their stylistic and graphical elements. All of these are concerned with creating presence, substituting textual features for face-to-face contact, or in creating and maintaining goodwill.

When we consider their dialogic dimension, we take into account reader expectations, which are constrained by assumptions about turn taking in conversations, by generic and organizational conventions, by situational constraints, and by hierarchical and organizational relationships. It should be obvious that relational and dialogic dimensions of correspondence overlap each other: it is not wise to attempt to parcel considerations out to these two categories as though they were sealed containers.

Given these dimensions, look at the following letters:

Two ancient letters from Egypt
a modern directive letter.