Before running X-12-ARIMA (or X-12 for short), you must have your data in a form that X-12 can read, and you must create an input specifications file (or "spec file"). This document will explain the basics of X-12 data formats and of writing a spec file. You will find more details in the X-12-ARIMA Reference Manual (US Census Bureau, 2002) and the X-12-ARIMA Quick Reference (US Census Bureau, 2002).