length vs prolongation

length and prolongation both are nouns.

Word	Noun	Adjective	Verb	Adverb
length	Yes	No	No	No
prolongation	Yes	No	No	No

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

length (noun)	prolongation (noun)
a section of something that is long and narrow	the act of prolonging something
the linear extent in space from one end to the other; the longest dimension of something that is fixed in place	amount or degree or range to which something extends
continuance in time	the consequence of being lengthened in duration
the property of being the extent of something from beginning to end
size of the gap between two places