A blue or magenta number pair shown near airspace boundaries (e.g., '100/80' or '55/SFC') represents:
Why →
Number pairs on sectional airspace boundaries represent the ceiling and floor of that airspace segment, always in hundreds of feet MSL. '100/80' means ceiling 10,000 feet MSL, floor 8,000 feet MSL. '55/SFC' means ceiling 5,500 feet MSL, floor at the surface. This pattern applies to Class B shelves, Class C shelves, and some Class D configurations.The trap →
Number pairs can look like frequencies to a new reader. The slash and the hundreds-of-feet convention identify them.SOURCE → FAA Aeronautical Chart User's GuideCHECKED APR 21ACS II.A.K1EASY