Not sure where to find this (or if it's been addressed previously), but regarding the 24 Hour Prior to Flight Time Check In rule, is this a US or UA thing only ? or does it vary from airline to airline/ or status ? Not a real important issue, but just this week I was able to check into my r/t flight on IB MAD-BIO (out Sat, return Sun) on the Wednesday before, yet my UA flight EWR-MAD tonite (Fri) I had to wait until Thurs evening, and can't check into my return until Sunday morining. I generally fly UA, so it may be just United ??? Any clarification would be appreciated