> For the complete documentation index, see [llms.txt](https://mandober.gitbook.io/math-debrief/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mandober.gitbook.io/math-debrief/600-toc/622-syntax/syntax.md). # Syntax > The *syntax* of a programming language is the set of rules that defines the combinations of symbols that are considered to be *expressions* (correctly structured statements) in that language. This applies equally to programming languages, where the document represents the source code, and to markup languages, where the document represents data. In a programming language, an *expressions* is a correctly structured statement (in that language). The syntax of a language defines its *surface form*. Text-based programming languages are based on sequences of characters, while visual programming languages are based on the spatial layout and connections between symbols. Documents that are *syntactically invalid* are said to have a *syntax error*. Syntax refers to the form of the code, and is contrasted with semantics, the meaning. During the processing of a programming language's document (source code), semantic processing generally comes after syntactic processing; however, in some cases, semantic processing is necessary for complete syntactic analysis. In compilers, the syntactic analysis is traditionally located in a compiler's frontend, while the semantic analysis comprises a compiler's backend. Computer language syntax is generally distinguished into 3 levels: 1. *Words* : the lexical level, determining how characters form tokens 2. *Phrases*: the grammar level, determining how tokens form phrases 3. *Context*: determines what objects or var names refer to Distinguishing in this way yields *modularity*, allowing each level to be described and processed separately and often independently. First, a *lexer* turns a linear sequence of characters into a linear sequence of *tokens*; this is known as *lexical analysis* (lexing). Second, a *parser* turns a linear sequence of tokens into a hierarchical syntax tree; this is known as *parsing* (in a narrow sense). Third, the *contextual analysis* resolves names and checks types. This modularity is sometimes possible, but in many real-world languages an earlier step depends on a later step; for example, the lexer hack in C is because tokenization depends on context. Even in these cases, syntactical analysis is often seen as approximating this ideal model.