1. INTRODUCTION

2. DATA

2.1. XMM-Newton Source Catalog

2.2. Gaia EDR3 Crossmatch

2.3. Object Classes and Their Catalogs

2.4. SRG/eROSITA eFEDS Catalog

2.5. SDSS-V (SDSS DR18) Crossmatch

3. THE X-RAY MAIN SEQUENCE

5. DISCUSSION

5.1. How to Interpret This Diagram

5.2. Why This Diagram Works: The Saturation Line

5.3. Earlier Versions of This Diagram

5.4. Applications to Upcoming Large X-ray and Optical Surveys

6. CONCLUSION