Date: 201911
Local time of day: 1201-1800
Location: DEN.Airport, CO
Flight conditions: VMC
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Human-Machine Interface; Confusion; Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Airspace Violation All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Chart Or Publication; Human Factors
Primary problem: Chart Or Publication

Narrative: Inbound to DEN Runway 35R; we were cleared direct to FRONZ (FAP on the ILS) for the visual approach about 30 miles or so SSW of the field. Captain set 7;000 feet for the altitude at FRONZ. We both pulled up our Class B reference charts for Denver to make sure we would remain in the Class B for the approach. We were descending through about 11;000 feet at the time and it looked like the only altitude that would be a concern on the way to 7;000 feet was the 9;000 feet floor at 20 DME. We made that crossing and headed down to 7;000 feet. Both of us thought that the next shelf had a base of 6;500 feet. We got down to 7;000 feet and we looked pretty low for how far out we were. I looked at the Class B reference chart again and saw we were in a sector with only an 8;000 feet base. The 6;500 feet base was for a tiny sector closer to the airport.The marking on the Jeppeson chart is very misleading. It's showing the altitudes for another sector that the actual text is not in. It has a tiny line drawn up to the small sector which is easy to miss with all of the other lines going every which way on the chart. The actual altitude for the sector we were in is not centered and not easily readable.Redesign the Class B reference page with an easier to read altitude for the small sector south of the airport. Perhaps even a numbered ball note would help.


Second narrative: Flew below the lower limit of the Class B Airspace while on a Visual Approach into DEN. While on arrival to DEN Runway 35R; ATC issued a clearance direct FRONZ (FAP on the ILS) and cleared the visual approach. Approximately 30 miles from the airport; (I) Captain set 7;000 feet for the altitude at FRONZ crossing. Both the FO (first Officer) and I selected the Class B reference charts for Denver; 10-1B; and observed our geolocation symbology. While descending through 11;000 feet; we correctly interpreted the limiting altitude of 9;000 feet between D30/DEN and D20/DEN but incorrectly interpreted the 8;000 feet limit between D20 and D10. Crossing the D20/DEN limit at 9;000 feet. I selected open descent to 7;000 feet. Traveling approximately 8 miles to achieve the descent to 7;000 feet; it appeared that we [were] relatively low based upon our distance remaining to the runway. Reselecting the 10-1B page; we recognized our error interpreting the limit (8;000 feet at DEN/D20) which was depicted east of our path and the V366 airway marked on the chart vice the limit of 6;500 feet which was along our path (marked inside the D20/DEN ring but applicable to the D10/DEN ring).I believe this error occurred due to the manner in which the chart is depicted. Upon closer scrutiny; the D10/DEN limit is depicted with a line that points to the interior area of a small portion of the Class B airspace but is placed in the D20/DEN ring. Our path to FRONZ was west of V389 and our geolocation symbol led us to a limit that appeared along our vector. However; it was not applicable to our actual distance from the airport.Redesign the Class B 10-1B page with an easier to read altitude for the small sector south of the airport. Additionally; I note that on other airport Class B charts; altitude limitations that are depicted outside of the boundary are shown with an arrow head pointing into the limited segment. On the DEN 10-1B page; the arrow head is not depicted.


Synopsis: A321 flight crew reported inadvertently flying below the floor of the DEN Class B airspace sector due to unclear charting.