15-Letter Words Matching I_____T________

There are 18 15-letter words that match I_____T________. 6 are common words, such as IMPENETRABILITY and INFINITESIMALLY. A rarer one is INSTANTANEITIES. 11 words contain a doubled letter.

Full list 18

impenetrabilityinfinitesimallyinfrastructuresinscrutablenessinstantaneouslyinsubstantiallyimpracticalnessindomitablenessinfrastructuralinterstratifiedinterstratifiesintractablenessidiomaticnessesillimitablenessimmunotherapiesindubitablenessinheritablenessinstantaneities

About this list

Words
18 (6 common words)
Common example
impenetrability
Rare example
instantaneities
Doubled letters
11
Ending in S
12

Questions

How many 15-letter words matching i_____t________ are there?

18. 6 of them are common words.

What is a common example?

IMPENETRABILITY is among the common words in this set.