Downstair vs. Downstairs — What's the Difference?

"Downstair," although similar in appearance to "downstairs," is seldom used and often considered incorrect or a typographical error in contemporary English. It lacks the plural form that signifies the area or concept related to the lower level of a structure. On the other hand, "downstairs" is the correct term used to describe both the location within a building (the lower floor or levels) and the direction towards that area. It can function as an adverb, adjective, or noun, providing versatility in its usage.

When used as an adverb, "downstairs" describes the action of moving to the lower level of a building, as in "going downstairs for breakfast." Conversely, "downstair" does not conventionally function in this capacity due to its rarity and perceived incorrectness. As an adjective, "downstairs" can describe a location on the lower floor ("the downstairs bathroom"), a usage for which "downstair" is not applied.

As a noun, "downstairs" denotes the lower floor or floors of a building, illustrating its broader applicability compared to the nonstandard "downstair." For instance, one might say, "The living room is downstairs," to indicate the room's location within a building. The plural form in "downstairs" inherently communicates the concept of multiple steps or a collection of rooms at a lower level, something that "downstair" fails to convey accurately.

In contemporary usage, "downstairs" is universally recognized and understood, reflecting its correct application in referring to anything related to the lower part of a building. "Downstair," due to its limited and often incorrect use, does not share this level of acceptance or utility in modern English, highlighting the importance of using "downstairs" for clarity and correctness.