International standard ISO 7736 defines a standard size for car audio head units and enclosures. The standard was originally established by the German standards body "Deutsches Institut für Normung" as DIN 75490, and is therefore commonly referred to as the "DIN car radio size". It was adopted as an international standard in 1984. 

Head units generally come in either "single DIN" (180 x 50 mm panel) or "double DIN" (180 x 100 mm panel) size. The depth is not standardized; as a result, some cars such as the Opel Manta / Ascona have the correct sized front aperture but will accommodate few DIN sized radios other than the original due to the shallow depth; this despite the vehicle being manufactured as late as 1988. The US standard for a DIN radio is 7" x 2" (although the actual 180 mm width converts to something like 7-3/32" so most people use 7-1/8" to allow for clearance) and the Double DIN sized radio is a 7" x 4". Some radios in Japanese Kei cars do not conform to the DIN standard however. 

For removing the unit, a pair of U-shaped devices are often used. The devices are inserted in the two pairs of holes, at either end of the stereo fascia, the action releasing the unit from the mounting and providing a pair of handles to pull the unit free. These tools vary from manufacturer to manufacturer. Becker uses a pair of key-shaped removal tools, with one hole each. Often a set of thin screwdrivers will work just as well.
What was a car audio head unit standard adopted?
Is it internationally recognized?
Which country created the standardization/
Which organization, specifically?
What is the standard commonly referred to?
And its formal/technical name?
What is the international standard known as today?
How many sizes are commonly used?
What are they?
What is the US standard?
What about the double DIN size?
Which area's cars don't use this standard?
Which cars are they?
What tools allow you to remove a head unit?
Are they universally identical?
If you didn't have access to those tools, what else could you use?
How many holes does the Becker style of tool use?
in 1984
in 1984
yes
 It was adopted as an international standard in 1984. 
Germany
established by the German standards body
"Deutsches Institut für Normung"
 by the German standards body "Deutsches Institut für Normung"
"DIN car radio size"
"DIN car radio size"
DIN 75490
as DIN 75490
International standard ISO 7736
International standard ISO 7736 
two
Head units generally come in either "single DIN" (180 x 50 mm panel) or "double DIN" (180 x 100 mm panel) size.
"single DIN" (180 x 50 mm panel) or "double DIN" (180 x 100 mm panel) size.
 "single DIN" (180 x 50 mm panel) or "double DIN" (180 x 100 mm panel) size.
7" x 2"
The US standard for a DIN radio is 7" x 2"
7" x 4"
he Double DIN sized radio is a 7" x 4".
Japan's
Some radios in Japanese Kei cars
Kei
Japanese Kei
a pair of U-shaped devices
For removing the unit, a pair of U-shaped devices are often used
no
 These tools vary from manufacturer to manufacturer.
a set of thin screwdrivers
Often a set of thin screwdrivers will work just as well.
one hole on each pair
Becker uses a pair of key-shaped removal tools, with one hole each