Modular design is a key challenge for enabling large-scale reuse of hardware modules. Unlike software, however, hardware designs correspond to physical circuits and inherit constraints from them. Timing constraints–which cycle a signal arrives, when an input is read–and structural constraints–how often a multiplier accepts new inputs–are fundamental to hardware interfaces. Existing hardware design languages do not provide a way to encode these constraints; a user must read a module’s implementation to understand how to use it. We present Filament, a language for modular hardware design that supports the specification and enforcement of timing and structural constraints. Filament uses timeline types, which describe the intervals of clock-cycle time when a given signal is available or required. Filament enables safe composition of hardware modules, ensures that the resulting designs are correctly pipelined, and predictably lowers them to efficient hardware.