What kind of policies are you looking for?
United:
https://www.united.com/CMS/en-US/tra...eCarry-On.aspx
Delta:
https://www.delta.com/content/www/en.../carry-on.html
American:
https://www.aa.com/i18n/travel-info/...on-baggage.jsp
JetBlue:
JetBlue Help Bad Input on Previous Form)
SouthWest:
https://www.southwest.com/html/custo...age/index.html
For example: "You may carry a violin, guitar or other small musical instrument on board as part of your carry-on baggage allowance as long as the instrument is placed in a hard case and there is space in the overhead bins or under the seat in front of you when you board the aircraft."
Seems less than vague.