Detailed explanation
Both between and among express relationships involving more than one person or thing, but they organise those participants differently. Use between when the entities are treated as separate and identifiable. This remains true with three or more participants: a treaty can be negotiated between five named states because each state is considered individually. Use among when people or things are viewed collectively as members of a group, without focusing on a separate relationship with every member.
The contrast is therefore not simply “two versus more than two”. That shortcut works in many elementary examples, but it fails in formal writing. Between is natural with named organisations, clearly delimited options, or reciprocal arrangements: communication between the departments, differences between the four models, an agreement between the partners. Among is natural with distribution, membership, shared feelings, and movement within a group: concern among staff, distribute the forms among participants, popular among younger readers.
Meaning can change with the preposition. “The negotiations between the delegates” highlights interactions involving distinct delegates, whereas “disagreement among the delegates” presents disagreement as a feature within the group. Similarly, “divide the work between the three researchers” may suggest three separately allocated portions, while “share the material among the researchers” focuses on distribution across the group.
Fixed expressions also matter. We say distinguish between, choose between, a link between, and between you and me. Common expressions with among include among other things, among the most important, and first among equals. Register does not determine the choice by itself; the writer’s conceptualisation does.
A useful self-check is to ask whether you could name the entities and describe the relationship separately. If yes, between is often appropriate. If the sentence presents a collective setting, membership, or broad distribution, among is usually better. In borderline cases, test both versions and identify the difference in focus rather than assuming that only the number of participants matters.
How to form it
Use between + plural or coordinated noun phrase when the participants are separate, identifiable entities: between the council, the union and the ministry. Use among + plural collective noun phrase when the participants are viewed as a group: among the delegates. After divide, distribute and share, choose the preposition according to whether the allocation is treated as separate portions or broad distribution.