plain vs flat

plain and flat both are nouns.

Word	Noun	Adjective	Verb	Adverb
plain	Yes	Yes	Yes	Yes
flat	Yes	Yes	No	Yes

As nouns, flat is a hyponym of plain; that is, flat is a word with a more specific, narrower meaning than plain:

Other hyponyms of plain include flood plain, floodplain, llano, moor, moorland, peneplain, peneplane, snowfield, steppe, tundra.

plain (noun)	flat (noun)
a basic knitting stitch made by putting the needle through the front of the stitch from the lefthand side	a suite of rooms usually on one floor of an apartment house
extensive tract of level open land	scenery consisting of a wooden frame covered with painted canvas; part of a stage setting
	a deflated pneumatic tire
	freight car without permanent sides or roof
	a shallow box in which seedlings are started
	a musical notation indicating one half step lower than the note named
	a level tract of land

plain (adjective)	flat (adjective)
not elaborate or elaborated; simple	(of a musical note) lowered in pitch by one chromatic semitone
lacking patterns especially in color	lacking contrast or shading between tones
free from any effort to soften to disguise	not reflecting light; not glossy
clearly revealed to the mind or the senses or judgment	having a surface without slope, tilt in which no part is higher or lower than another
not mixed with extraneous elements	horizontally level
lacking embellishment or ornamentation	commercially inactive
lacking in physical beauty or proportion	lacking the expected range or depth; not designed to give an illusion or depth
	having lost effervescence
	having a relatively broad surface in relation to depth or thickness
	lacking taste or flavor or tang
	flattened laterally along the whole length (e.g., certain leafstalks or flatfishes)
	stretched out and lying at full length along the ground
	sounded or spoken in a tone unvarying in pitch
	not modified or restricted by reservations
	lacking stimulating characteristics; uninteresting

plain (verb)	flat (verb)
express complaints, discontent, displeasure, or unhappiness

plain (adverb)	flat (adverb)
unmistakably (`plain' is often used informally for `plainly')	in a forthright manner; candidly or frankly
	with flat sails