Boxing and professional combat sports in general, pairs contestants based on their weight. Prior to the fight, contestants will step on a scale, sometimes removing all their clothing, to affirm that they are under the maximum allowed weight for their particular weight class.
The weigh-in itself is a sight to behold in professional boxing. It is part of the ritual of boxing and even heavyweights, who have no limit on their weight, still participate in the practice. The weigh-in itself consists of the two competing boxers entering a staged room with a scale in the center the day before a fight and being weighed. The fighters are usually accompanied by their entourage as well as a host of media, paparazzi and the official weigh-in staff. Some of the more bold and brash boxers will take the opportunity to trash talk their opponent. At the end, both boxers typically stand face-to-face with their fist raised before returning to their pre-fight routine.
There are some situations where the weigh-in structure is modified. One example of this is in the event of a tournament. Multi-day boxing tournaments enforce general and daily weigh-ins. The general weigh-in occurs at the beginning of the tournament and both the upper and lower weight limits are enforced, but for daily weigh-ins, only the upper limit is enforced. In the event a boxer fails to make weight at a general weigh-in, the boxer may be reassigned to another weight class if there is room in the bracket. Failing a daily weigh-in, however, will result in immediate disqualification.
Other organizations, like the IBO, can also enforce extraneous rules on weight, restricting the difference between two boxers weights. For example, if two boxers are considered middleweight fighters (weight limit 160lbs) the difference between the weight of the two boxers can be no more than ten pounds.