It's been a long time since I checked a bag, so I don't know for sure, but in the distant past airlines put sticky tape on one of the walls of the scale thus providing a silhouette of a suitcase.. The tape approximated the maximum size allowed, and I guess assumed a standard width for the third of the three measures. If the bag was obviously too big, it got noticed. Southwest doesn't nickel and dime you. Thus I suspect that if your bag is an inch too big, you'll be fine.
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