The answers to the clues below will have something in common.

Site of US Capitol
Former Princess of Wales
A metric measure  (two answers)
Host
Physician
US State Postal abbreviation


HINT:
It has to do with numbers.

Answers:

Each clue can be answered using Roman Numerals:

Site of US Capitol - Washington DC
Former Princess of Wales - Princess DI(ana)
Metric measures - CC or ML
Host - MC (emcee)
Physician - MD
Michigan -  MI 

Credit: NY Times Crossword - Aug. 18, 1985
Charles M. Deber
WHEN IN ROME  (adapted)