Instrumental Relationships
Instrumental relationships are based on what people can do for us - and what we can do for them - in particular situations. In this respect, instrumental relationships are often characterised as being rational, calculated, relationships that are largely devoid of the emotion associated with affective relationships.
An example
here might be the relationship you have with a check-out operator at a supermarket. This
is a rational relationship (based on an exchange of goods - you hand over
money and the check-out operator gives you the things you've just paid-for) and, in a
sense, highly calculated (you do something in the rightful expectation of
receiving something in exchange, for example).
Although the relationship may be "pleasant" (in the sense you may exchange a greeting with the person serving you), instrumental relationships have no great emotional depth (which is not surprising since they are designed to be that way - life would be impossible if we had to deal with everyone we met as if they were a close personal friend or relation)
Instrumental relationships, as you might expect, become highly-developed in modern societies when people start to live-out substantial parts of their lives in the public sphere.