Adam Clarke on Judges 19:29

Divided her - into twelve pieces - There is no doubt that with the pieces he sent to each tribe a circumstantial account of the barbarity of the men of Gibeah; and it is very likely that they considered each of the pieces as expressing an execration, "If ye will not come and avenge my wrongs, may ye be hewn in pieces like this abused and murdered woman!" It was a custom among the ancient Highlanders in Scotland, when one clan wished to call all the rest to avenge its wrongs, to take a wooden cross, dip it in blood, and send it by a special messenger through all the clans. This was called the fire cross, because at sight of it each clan lighted a fire or beacon, which gave notice to all the adjoining clans that a general rising was immediately to take place.