Common ground is what both parties know and know that the other knows

What it is

Common ground is not merely information that two people happen to know. It is knowledge, beliefs, and assumptions that interlocutors have reason to treat as mutually shared. It includes cultural or joint experience available before an exchange and contributions accepted during the current conversation. Its defining property is the judgment of mutual availability: speakers use it to decide what can remain implicit, and addressees use it to resolve expressions such as “that version” or “the original plan.”

Why it happens

Joint activity requires people to anticipate how partners will interpret instructions and act, which depends on a model of the partner's knowledge. Public joint experience, a mutually visible setting, and accepted dialogue provide evidence of sharing. As common ground accumulates, partners can reuse names and references, reducing descriptive work. Coincidentally knowing the same fact is insufficient when neither has evidence that the other knows it. Everyday communication relies on sufficiently strong evidence, not literal infinite recursion through one another's mental states.

Studying it

Referential communication tasks assign a director to describe hard-to-name figures and a matcher to identify them over repeated rounds. Measures include description length, completion time, selection errors, clarification turns, and stabilization of shared labels. Manipulating visual co-presence, prior joint experience, or partner-visible information tests what speakers treat as common ground. The relevant unit is the dyad plus its interaction history; an individual's memory score is not itself a measure of common ground.

Where it stops holding

Common ground is a working assumption, not an objective intersection obtained by reading another mind. Partners may disagree about what has been established. Culture, group membership, and access permissions provide evidence of sharing but do not guarantee that a particular person knows a fact. High-consequence work requires explicit readback or confirmation when plausible mutuality is not enough.

Applying it

• Keep decisions, term definitions, object versions, and current goals in a mutually visible, addressable place rather than only in personal histories.
• Distinguish delivered, viewed, and understood states; request stronger evidence only where task risk warrants it.
• Give newcomers access to the decisions that produced the current state instead of making them infer it from a complete message stream.
• Pair established and new collaborators on a reference or handoff task and measure clarification turns and reference errors.

Related

• Same group: V1.03.2 Common ground is built through reciprocal confirmation, not one-way transmission · V1.03.3 Feedback cost shapes confirmation · V1.03.4 Insufficient common ground produces divergent interpretations · V1.03.5 Experts' omissions do not work for newcomers
• Adjacent: V2.04 Shared reference · V4.05 Handoffs and context transfer
• Search terms: common ground · mutual knowledge · referential communication