When looking for a NYC apartment, it helps to know what you want.

Even if you already know the type of building you’re looking for (a condo, co-op, brownstone or townhouse), you still may run into terms you’re not familiar with. Some insight on a few:

Popular with families for the space and privacy they afford, duplex and triplex apartments occupy two or three adjoining floors connected by private interior stairs. Sometimes separate apartments on adjoining floors are converted to a duplex or triplex by the owner. A pair of apartments on adjoining floors is currently on the market at the thoroughly modern, architecturally significant Richard Meier-designed tower at 173 Perry Street, where apartments are known for their floor-to-ceiling windows and Hudson River views.

The penthouse apartment concept was born in NYC in the “roaring ‘20s,” when demand for high-end city living led to the creation of luxury living quarters on top floors of buildings. One of the first–and most famous–penthouse apartments in NYC was the duplex atop 1040 Park Avenue owned by publisher Condé Nast.

An NYC loft is usually either an apartment in a formerly commercial or industrial building converted into a residence, or an apartment created to look like one, with an open layout and few internal walls, often with high ceilings and oversized floor-to-ceiling windows. You’ll find a list of the New York City’s top ten loft buildings here. Technically, a floor-through apartment means the unit occupies the entire floor, which affords an unusual level of privacy and space. It can also refer to an apartment that runs from front to back on a floor, with two apartments per floor.

A parlor (or parlor floor) apartment refers to an apartment one level up from street level. A garden apartment is located on the ground floor–sometimes partially below street level and down a few steps–with access to private outdoor space such as a backyard or patio. A maisonette is an apartment with its own street-level entrance, a reminder of an era when the butler would answer the door. A covetable maisonette in the elegant pre-war co-op at 998 Fifth Avenue even has its own private address at 999 Fifth Avenue.

Find out more about the city’s best buildings with our directory of New York City condo and co-op apartments.