Question: Where are nets usually placed in association football?
About the article "Association football", section "Pitch".

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Drift netting is a fishing technique where nets, called drift nets, hang vertically in the water column without being anchored to the bottom.
The nets are keep vertical in the water by floats attached to a rope along the top of the net and weights attached to another rope along the bottom of the net.
Drift nets generally rely on the entanglement properties of loosely affixed netting.
Folds of loose netting, much like a window drapery, snag on a fish's tail and fins and wrap the fish up in loose netting as it struggles to escape.
However the nets can also function as gill nets if fish are captured when their gills get stuck in the net.
The size of the mesh varies depending on the fish being targeted.
These nets usually target schools of pelagic fish.
Traditionally drift nets were made of organic materials, such as hemp, which were biodegradable.
Prior to 1950, nets tended to have a larger mesh size.
The larger mesh only caught the larger fish, allowing the smaller, younger ones to slip through.
When drift net fishing grew in scale during the 1950s, the industry changed to synthetic materials with smaller mesh size.
Synthetic nets last longer, are odourless and may be nearly invisible in the water, and do not biodegrade.
Most countries regulate drift net fisheries within their territories.
Such fisheries are also often regulated by international agreements.
Drift net fishing became a commercial fishing practice because it is cost effective.
Nets can be placed by low-powered vessels making it fuel efficient.
Drift nets are also effective at bringing in large amounts of fish in one catch.
Prior to the 1960s net size was not limited, and commercially produced nets were commonly as long as .
In 1987 the U.S. enacted the Driftnet Impact, Monitoring, Assessment and Control Act limiting the length of nets used in American waters to 1.5 nautical miles (~1.7 miles, ~2.778 km).
In 1989 the United Nations General Assembly (UNGA) placed a moratorium on the practice of drift net fishing.
In 1992 the UN banned the use of drift nets longer than 2.5 km long in international waters.  
Fishing nets are meshes usually formed by knotting a relatively thin thread.
About 180 AD the Greek author Oppian wrote the "Halieutica", a didactic poem about fishing.
He described various means of fishing including the use of nets cast from boats, scoop nets held open by a hoop, and various traps "which work while their masters sleep".
Netting is the principal method of commercial fishing, though longlining, trolling, dredging and traps are also used.
Artisanal techniques * Chinese fishing nets - are shore operated lift nets.
Huge mechanical contrivances hold out horizontal nets with diameters of twenty metres or more.
The nets are dipped into the water and raised again, but otherwise cannot be moved.
* Lampuki nets - are an example of a traditional artisanal use of nets.
Since Roman times, Maltese fishers have cut the larger, lower fronds from palm trees which they then weave into large flat rafts.
The rafts are pulled out to sea by a luzzu, a small traditional fishing boat.
In the middle of the day, lampuki fish (the Maltese name for mahi-mahi) school underneath the rafts, seeking the shade, and are caught by the fishers using large mesh nets.
Cast nets - are round nets with small weights distributed around the edge.
They are also called "throw nets."
The net is caste or thrown by hand in such a manner that it spreads out on the water and sinks.
Fish are caught as the net is hauled back in.
This simple device has been in use, with various modifications, for thousands of years.
Drift nets - are nets which are not anchored.
They are usually gillnets, and are commonly used in the coastal waters of many countries.
Their use on the high seas is prohibited, but still occurs.
Ghost nets - are nets that have been lost at sea.
They can be a menace to marine life for many years.
Gillnets - catch fish which try to pass through by snagging on the gill covers.
Trapped, the fish can neither advance through the net nor retreat.
Hand nets - are small nets held open by a hoop.
They have been used since antiquity.
They are also called scoop nets, and are used for scooping up fish near the surface of the water.
They may or may not have a handle–if they have a long handle they are called "dip nets".
When used by anglers to help land fish they are called "landing nets".
Because hand netting is not destructive to fish, hand nets are used for tag and release, or capturing aquarium fish.
Seine nets - are large fishing nets that can be arranged in different ways.
In purse seining fishing the net hangs vertically in the water by attaching weights along the bottom edge and floats along the top.
Danish seining is a method which has some similarities with trawling.
A simple and commonly used fishing technique is beach seining, where the seine net is operated from the shore.
Surrounding nets -  Tangle nets - also known as "tooth nets", are similar to gillnets except they have a smaller mesh size designed to catch fish by the teeth or upper jaw bone instead of by the gills.
Trawl nets - are large nets, conical in shape, designed to be towed in the sea or along the sea bottom.
The trawl is pulled through the water by one or more boats, called trawlers.
The activity of pulling the trawl through the water is called trawling.  
There are a number of nets, usually anywhere between two and six.
This is a game that is generally more fun with more people.
Teams may consist of a few or many players, depending on how many players are taking part.
The objective is to score a set number of goals (usually three or five) into one of the opposing teams' nets; when this happens, that team is out until the next round begins.
The team which wins the most rounds are declared the overall winners.
There is rarely a time limit, and the round will continue until one team remains.
There are a number of variable rules, such as Handball Nets, which means the Handball rule in football is reversed, and a foul is committed if a foot touches the ball.
The game can be played anywhere, with any sport that involves nets, and in almost any space.
That is why it is one of the most popular forms of street football. 
Rules of the game.
The field.
Arena football is played exclusively indoors, in arenas usually designed for either basketball or ice hockey teams.
The field is the same width () and length () as a standard NHL hockey rink.
The scrimmage area is 50 yards long (unlike the field in NFL which is 100 yards long), and each end zone is approximately eight yards.
Depending on the venue in which a game is being played, the end zones may be rectangular (like a basketball court) or, where necessary because of the building design, curved (like a hockey rink).
Each sideline has a heavily padded barrier, with the padding placed over the hockey dasher boards.
The goalpost uprights are wide, and the crossbar is above the playing surface.
Taut rebound nets on either side of the posts bounce any missed field goals back into the field of play.
The ball is "live" when rebounding off these nets or their support apparatus.
The entire goalframe and goalside rebound net system is suspended on cables from the rafters.
The bottom of the two goalside rebound nets are off the playing surface.
Each netframe is high by wide.
A player is not counted as out of bounds on the sidelines unless he is pushed into or falls over the sideline barrier.
This rule was put in place before the 2006 season.
Before that time, a sideline with only a small amount of space (typically 6" to 12") existed between the sideline stripe and the barrier which would provide the space for a ball carrier to step out of bounds before hitting the sideline barrier. 
In standard outdoor American football, the team awarded the touchback receives possession of the ball at its own 20-yard line, except in college football, where the team awarded the touchback receives possession at its own 25-yard line on kickoffs and free kicks after a safety as of the 2012 season.
In arena football, and other indoor football games, a touchback results in the team awarded the touchback receiving the football at its own 3-yard line.
This can result from any of the above events except for punting, which is not a part of arena football.
(In arena football, a kicked ball usually bounces back into play off of the rebound nets, but the above can still occur when the ball lands in the slack nets behind the goalposts after a kickoff, passes under the rebound nets and out of play, or in the event of fumbles and interceptions.)