How do I write a text alternative for a diagram with labeled parts?

Describe the labeled parts in relation to one another. A label list names the pieces, but may leave readers unsure which part points to, contains, connects with, or comes before another. Begin by identifying what the diagram depicts, then state only the relationships visibly represented.

For example, suppose the supplied diagram shows three labeled boxes connected by arrows. A list might say “Intake; review; response.” A more useful description would say: “The diagram shows a three-step process. An arrow leads from Intake to Review, and a second arrow leads from Review to Response.” This describes the depicted arrangement without claiming what the steps mean beyond their labels.

W3C’s Web Accessibility Initiative recommends a short identification plus a longer textual representation when substantial information depends on understanding a complex image. A longer description can communicate visual relationships and may use headings or a table. [S1] The diagram itself—not that general guidance—determines which labels and connections to include.

Make the short identification tell readers what kind of diagram they are encountering and its subject, if that is clear from the image or accompanying text. For instance, “Diagram of labeled parts in a water-filter assembly” is a model to use only if that is what your image depicts. Don’t add a purpose, function, or conclusion that is not supplied.

Use a longer description when a short identification and surrounding text would leave out essential information conveyed by the diagram. WAI recommends a short identification alongside a longer representation of essential information for complex images. [S1] That guidance does not determine whether a particular diagram is complex or establish what it proves; assess the actual image and what readers need from it.

A practical test: if someone could not follow the diagram’s important labeled relationships from the short text alone, add a longer description. Keep it focused on the information the diagram conveys, rather than decorative details.

Use literal, visual language first: “A line connects A to B,” “A is inside B,” “C is positioned below A,” or “An arrow points from A toward B.” Name a direction or sequence only when the visual marks make it clear. Describe an arrow’s direction as drawn, but don’t call it “causes,” “sends data,” or “leads to a result” unless the supplied material establishes that meaning.

Here is a fill-in template:

“[Short identification of the diagram]. It shows [number or grouping of labeled parts, if useful]. [Part A] is [visibly shown relationship] to [Part B]. [Line, arrow, boundary, or other visible connector] links [part] with [part] and points [direction, if shown]. [Add another essential relationship]. The diagram does not specify [important point that might otherwise be assumed].”

The last sentence is optional. Use it when a likely interpretation goes beyond what the diagram states. Replace every bracket with information checked against the image; delete any sentence that doesn’t apply.

Compare the draft with the diagram label by label: confirm names, positions, connections, and any stated order. Remove interpretations the image does not support. If the description has several distinct groups or relationships, separate them with short headings or use a simple text list. WAI notes that a longer description can be structured with headings or a table; choose a format that makes the depicted relationships clear. [S1]

Assuming a supplied diagram shows two labeled components joined by a line, a cautious draft could be: “The diagram labels two components, A and B. A line connects A to B. No direction is marked.” That wording reports only the assumed visible details. In an actual description, substitute the real labels and check that the line has no directional marker.

Before publishing, check whether readers can identify the diagram’s subject, locate each important label, and understand the connections or sequence visibly shown. A text description cannot establish facts missing from the image or resolve an ambiguous symbol; consult the diagram’s author or another authoritative source if that information matters.