Who began the use of alerted undergarments and ballet slippers during the neoclassical period

your word would be isle.

isle is an island or peninsula especially a small one.

if you add the letter "a" you get aisle, which is a passage between 2 rows of something like chairs, shelves, or something else.

the answer of "what has three legs but never walks" is a yard stick. this answer can be found at