# State diagrams > "A state diagram is a type of diagram used in computer science and related fields to describe the behavior of systems. State diagrams require that the system described is composed of a finite number of states; sometimes, this is indeed the case, while at other times this is a reasonable abstraction." Wikipedia Mermaid can render state diagrams. The syntax tries to be compliant with the syntax used in plantUml as this will make it easier for users to share diagrams between mermaid and plantUml. ```mermaid-example stateDiagram-v2 [*] --> Still Still --> [*] Still --> Moving Moving --> Still Moving --> Crash Crash --> [*] ``` Older renderer: ```mermaid-example stateDiagram [*] --> Still Still --> [*] Still --> Moving Moving --> Still Moving --> Crash Crash --> [*] ``` In state diagrams systems are described in terms of its states and how the systems state can change to another state via a transitions. The example diagram above shows three states **Still**, **Moving** and **Crash**. You start in the state of Still. From Still you can change the state to Moving. In Moving you can change the state either back to Still or to Crash. There is no transition from Still to Crash. ## States A state can be declared in multiple ways. The simplest way is to define a state id as a description. ```mermaid-example stateDiagram-v2 s1 ``` Another way is by using the state keyword with a description as per below: ```mermaid-example stateDiagram-v2 state "This is a state description" as s2 ``` Another way to define a state with a description is to define the state id followed by a colon and the description: ```mermaid-example stateDiagram-v2 s2 : This is a state description ``` ## Transitions Transitions are path/edges when one state passes into another. This is represented using text arrow, "\-\-\>". When you define a transition between two states and the states are not already defined the undefined states are defined with the id from the transition. You can later add descriptions to states defined this way. ```mermaid-example stateDiagram-v2 s1 --> s2 ``` It is possible to add text to a transition. To describe what it represents. ```mermaid-example stateDiagram-v2 s1 --> s2: A transition ``` ## Start and End There are two special states indicating the start and stop of the diagram. These are written with the [\*] syntax and the direction of the transition to it defines it either as a start or a stop state. ```mermaid-example stateDiagram-v2 [*] --> s1 s1 --> [*] ``` ## Composite states In a real world use of state diagrams you often end up with diagrams that are multi-dimensional as one state can have several internal states. These are called composite states in this terminology. In order to define a composite state you need to use the state keyword followed by an id and the body of the composite state between \{\}. See the example below: ```mermaid-example stateDiagram-v2 [*] --> First state First { [*] --> second second --> [*] } ``` You can do this in several layers: ```mermaid-example stateDiagram-v2 [*] --> First state First { [*] --> Second state Second { [*] --> second second --> Third state Third { [*] --> third third --> [*] } } } ``` You can also define transitions also between composite states: ```mermaid-example stateDiagram-v2 [*] --> First First --> Second First --> Third state First { [*] --> fir fir --> [*] } state Second { [*] --> sec sec --> [*] } state Third { [*] --> thi thi --> [*] } ``` *You can not define transitions between internal states belonging to different composite states* ## Choice Sometimes you need to model a choice between two or more paths, you can do so using <<choice>>. ```mermaid-example stateDiagram-v2 state if_state <> [*] --> IsPositive IsPositive --> if_state if_state --> False: if n < 0 if_state --> True : if n >= 0 ``` ## Forks It is possible to specify a fork in the diagram using <<fork>> <<join>>. ```mermaid-example stateDiagram-v2 state fork_state <> [*] --> fork_state fork_state --> State2 fork_state --> State3 state join_state <> State2 --> join_state State3 --> join_state join_state --> State4 State4 --> [*] ``` ## Notes Sometimes nothing says it better then a Post-it note. That is also the case in state diagrams. Here you can choose to put the note to the *right of* or to the *left of* a node. ```mermaid-example stateDiagram-v2 State1: The state with a note note right of State1 Important information! You can write notes. end note State1 --> State2 note left of State2 : This is the note to the left. ``` ## Concurrency As in plantUml you can specify concurrency using the -- symbol. ```mermaid-example stateDiagram-v2 [*] --> Active state Active { [*] --> NumLockOff NumLockOff --> NumLockOn : EvNumLockPressed NumLockOn --> NumLockOff : EvNumLockPressed -- [*] --> CapsLockOff CapsLockOff --> CapsLockOn : EvCapsLockPressed CapsLockOn --> CapsLockOff : EvCapsLockPressed -- [*] --> ScrollLockOff ScrollLockOff --> ScrollLockOn : EvScrollLockPressed ScrollLockOn --> ScrollLockOff : EvScrollLockPressed } ``` ## Setting the direction of the diagram With state diagrams you can use the direction statement to set the direction which the diagram will render like in this example. ```mermaid-example stateDiagram direction LR [*] --> A A --> B B --> C state B { direction LR a --> b } B --> D ``` ## Comments Comments can be entered within a state diagram chart, which will be ignored by the parser. Comments need to be on their own line, and must be prefaced with `%%` (double percent signs). Any text after the start of the comment to the next newline will be treated as a comment, including any diagram syntax ```mmd stateDiagram-v2 [*] --> Still Still --> [*] %% this is a comment Still --> Moving Moving --> Still %% another comment Moving --> Crash Crash --> [*] ``` ## Styling Styling of the a state diagram is done by defining a number of css classes. During rendering these classes are extracted from the file located at src/themes/state.scss