Q: Why is my note or if/else block throwing a syntax error?

The most common cause is a note ... : some text that wraps onto a second line -- single-line notes (note over A, B : text) must stay on one line; anything longer needs the block form (note over A, B / end note). For if/else, make sure every if opens with then (yes) or a similar label and is closed with a matching endif -- a missing endif is the next most common break.

Q: How do I change the color scheme or apply a built-in theme?

Add !theme right after @startuml to apply one of PlantUML's built-in themes (e.g. !theme plain, !theme cerulean, !theme sketchy-outline -- see the full list in PlantUML's theme gallery). For one-off tweaks instead of a whole theme, use skinparam directives like skinparam backgroundColor #FDFDFD or skinparam classBackgroundColor #LightBlue.

Question 1

What do @startuml and @enduml actually do?

Accepted Answer

They're the required wrapper around every PlantUML diagram -- @startuml marks the start of a diagram block and @enduml marks its end. Everything between them is parsed as diagram syntax; text outside the block is ignored. A single .puml file can contain multiple @startuml/@enduml blocks, and each renders as its own separate diagram image.

Question 2

How do I render a PlantUML diagram without pasting it into an online tool?

Accepted Answer

Run the plantuml.jar locally: java -jar plantuml.jar diagram.puml, which produces diagram.png next to the source file (add -tsvg for SVG output). It requires a Java runtime and, for some diagram types, Graphviz's dot on your PATH. Most editors (VS Code's PlantUML extension, IntelliJ) also render a live preview using the same jar or a bundled server.

Question 3

How do I switch between sequence, class, activity, and other diagram types?

Accepted Answer

What do @startuml and @enduml actually do?

How do I render a PlantUML diagram without pasting it into an online tool?

How do I switch between sequence, class, activity, and other diagram types?

PlantUML infers the diagram type from the syntax you use inside @startuml/@enduml -- there's no separate "mode" flag. Arrows like Alice -> Bob start a sequence diagram, class Foo { ... } and relationship arrows like <|-- start a class diagram, start / :Action; / stop start an activity diagram, and actor/usecase declarations start a use case diagram. You can't mix diagram types in a single @startuml block.

Why is my note or if/else block throwing a syntax error?

How do I change the color scheme or apply a built-in theme?

Question 4

Why is my note or if/else block throwing a syntax error?

Accepted Answer

The most common cause is a note ... : some text that wraps onto a second line -- single-line notes (note over A, B : text) must stay on one line; anything longer needs the block form (note over A, B / end note). For if/else, make sure every if opens with then (yes) or a similar label and is closed with a matching endif -- a missing endif is the next most common break.

Question 5

How do I change the color scheme or apply a built-in theme?

Accepted Answer

Add !theme <name> right after @startuml to apply one of PlantUML's built-in themes (e.g. !theme plain, !theme cerulean, !theme sketchy-outline -- see the full list in PlantUML's theme gallery). For one-off tweaks instead of a whole theme, use skinparam directives like skinparam backgroundColor #FDFDFD or skinparam classBackgroundColor #LightBlue.

Cheatsheet: PlantUML

See also

FAQ

What do @startuml and @enduml actually do?

How do I render a PlantUML diagram without pasting it into an online tool?

How do I switch between sequence, class, activity, and other diagram types?

Why is my note or if/else block throwing a syntax error?

How do I change the color scheme or apply a built-in theme?