plainer / planar / planer are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
plainer: (adjective) not elaborate or elaborated; simple. (adjective) lacking patterns especially in color. (adjective) not mixed with extraneous elements. (adjective) free from any effort to soften to disguise. (adjective) lacking embellishment or ornamentation.
planar: (adjective) involving two dimensions.
planer: NA
plainer: p·l·EY·n·er
planar: p·l·EY·n·er
planer: p·l·EY·n·er
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)