A mezzanine is a partial floor built between the main floors of a building, like a balcony that extends partway into a larger space. Picture the upper level in a two-story bookstore or library: if you're standing on the ground floor looking up, you might see a smaller floor halfway to the ceiling with a railing around it. That's a mezzanine. It's not a complete second story because it only covers part of the area below, leaving an open space where you can see all the way up to the ceiling.

Mezzanines appear in theaters, where wealthy patrons once sat in mezzanine seats above the main floor but below the highest balcony. Modern office buildings often include mezzanines to add workspace without the expense of constructing an entire additional floor. Some shopping malls use mezzanines to create extra retail space while keeping the airy, open feeling shoppers enjoy.

Architects and builders add mezzanines when they have high ceilings and want to maximize useful space. A clever mezzanine can transform a tall, empty room into something more practical while preserving the dramatic feeling of all that vertical space.