The transfer period describes the times during the year in which clubs can buy or acquire players who are attached to another football club to their own.
There are typically two transfer periods each year a longer one in the summer months following the end of one football season and the next, and a shorter one in January.
Click here for a link to the winter transfer period for many European clubs.
National football federations are allowed to set the exact dates during which the transfer period should run, but, according to FIFA rules, the longer period should not exceed 12 weeks, and the shorter one four weeks.
In reality, most European leagues, with a few exceptions, such as Turkey and Portugal, tend to open and close around the same dates. This helps teams who play in different competitions from being at a disadvantage from their continental competitors.
The United States and Canada follow similar rules, the only difference in their case is that the summer window occurs in the middle of their league seasons rather than at the end of it.
Players under contract are only meant to move in these transfer windows.
There are exceptions. One is that players who are out of contract are allowed to move at any time. Often known as free agents, they are players whose contract has ended at one club and technically are unemployed, with no current employer.
Sometimes players will allow their contracts to deliberately run down because then they can be signed by another club without the need for a transfer fee. And, because in some cases they can expect a substantial signing-on fee when they agree to join a new club, this can be a lucrative option for some.
The rules are also different for youth players. Youth players refer to those under 18 who have yet to sign their first professional contract. There have been several high profile cases about the recruitment and subsequent mistreatment of young players from Africa, resulting in tighter regulations for the safety of young players.
The FIFA Rules for Minor Players were created to protect young players from being taken advantage of.
However, whilst transfers between clubs is only supposed to occur in the two dedicated windows, it would be naïve to think that this is the only times during a year when players and their representatives discuss moves .
In reality, there is constant dialogue behind the scenes throughout the year between agents, and other representatives of players, and clubs who may want to recruit them.
Sometimes this is above board. Players whose contract are due to expire at the end of the season are allowed to begin negotiations with foreign clubs in January, six months before the contract expiration. They may even sign pre-contract agreements with them.
Other times such discussions take place in secret and are against the rules. Known as tapping-up, clubs and players can face fines and other punishments such as a transfer ban if caught.
However, it is notoriously difficult to prove, and few are ever brought to account for it.