Let’s begin with a few DOUBLETS. The adjective planus evolved into two English HOMONYMS, plain and plane, which both have clear semantic links to the meanings “level,” “even,” “flat.” Both plain and plane have various English uses, of course, and can be several different parts of speech. In Italian, where the initial pl– of Latin regularly changed to pi-, planus became piano, an adjective that still means “plain,” “level,” or “flat.” As an adverb, piano suggests “softly,” a meaning it carries as a musical notation. A keyboard instrument that could be played either softly or loudly (“strongly”) was called the pianoforte, which we have shortened to piano. An Italian derivative also provides one of the doublets from Latin rotundus, in the form of rotunda, a round building or a round room. The English words round and rotund are perfect doublets; rotund has become rather specialized in meaning, since we use it only to refer to human girth. Another pair of doublets are integer and entire (< OF entier).
The Latin adjectives pronus and supinus meant “leaning forward” and “bent backward”; and their English derivatives prone and supine still express the literal image of lying face down or face up, respectively. If you are prone to do something, you are eagerly leaning forward (in a metaphorical sense, at least); if your attitude is supine, you are sluggish and inert—flat on your back, so to speak.
In Latin, levis and gravis are exact opposites in meaning (ANTONYMS): both refer to physical weight, but both can be used also in a moral or ethical sense (like our “light” and “heavy” reading). The English derivatives levity and gravity are similarly flexible, but the adjective grave (= “serious,” “solemn”) is not applied to physical weight. This word grave, derived from L gravis, has no connection with the grave in which one is buried; that is a Germanic homograph (§12), from OE græf.