C4

C4Context
  title System context for internet banking
  Person(customer, "Banking customer", "A customer of the bank")
  Enterprise_Boundary(bank, "Bank") {
    System(banking, "Internet banking", "Lets customers view their accounts")
    System_Ext(mail, "E-mail system", "Microsoft Exchange")
  }
  Rel(customer, banking, "Uses")
  Rel(banking, mail, "Sends e-mail", "SMTP")

Syntax

• C4Context, C4Container, C4Component, C4Dynamic and C4Deployment are the five levels.
• Person, System, Container, Component and their _Ext, Db and Queue forms declare elements as (alias, label, description).
• Enterprise_Boundary, System_Boundary and Boundary group elements in { … }.
• Rel(from, to, label, technology) relates two; BiRel, Rel_U, Rel_D, Rel_L and Rel_R are its variants.