On a sectional chart, a solid blue line typically indicates the boundary of:

Why →Class B airspace is depicted on sectional charts with solid blue lines. Class C uses solid magenta lines, and Class D uses dashed blue lines.

The trap →Blue lines appear for both Class B (solid) and Class D (dashed). The color AND line style both matter. Solid blue = Class B.

Field note →Memory trick: Big airports (Class B) get Bold solid lines. Class D gets Dashed. Class C gets C-olored magenta.