In technical terms, Section 403 of the FAA Reauthorization Act of 2012 is now in full effect. What that means for travelers: As of Dec. 30, 2014, small musical instruments now qualify as a "carry-on" on any US airline. The St. Louis Post-Dispatch shares the American Federation of Musicians' statement on the change, which references the " very arbitrary and contradictory size and weight requirements" musicians formerly encountered and explains that instruments like violins and guitars can now be kept in overhead storage or under plane seats. Said storage is available on the same first-come, first-served basis as other baggage.
USA Today reports the rule "anticipated possible discord" with the scenarios that could follow a guitar taking up a space that could otherwise house multiple bags: Once the guitar is in the bin, its owner can't be made to remove it. As for larger instruments of up to 165 pounds (like tubas), passengers can also buy seats for them as long as the instruments are tied down and don't block emergency exits or signs. Airlines will have to spend an estimated $475,000 a year to train workers on the instrument policy. (YouTube star Dave Carroll of "United Breaks Guitars" fame should be pleased.)