Syntax famously consists of abstract hierarchical representations, essentially instructions for combining words into larger units like sentences. Less famously, most theories of syntax also assume a higher level of abstract representation. Representations at this level comprise instructions for creating the hierarchical representations used to create sentences. To date, however there is no experimental evidence for this additional level of abstraction. Here, we explain why the existence of such representations would imply that, under certain circumstances, speakers should be able to produce structures they have never been exposed to, and we test this prediction directly. We ask: Given the right type of input, can speakers learn a syntactic structure without direct exposure? In particular, different types of relative clauses have different surface word orders. These may be represented in two ways: with many individual representations or one general representation. If the latter, then learning one type of relative clause amounts to learning all types. We teach participants a novel grammar for only some relative clause types (e.g., just subject relative clauses) and test their knowledge of other types (e.g., object relative clauses). Across experiments, participants consistently produced untrained types, implicating the existence of this higher level of abstract syntactic knowledge.