The "Billboard" 200 is a record chart ranking the 200 most popular music albums and EPs in the United States. It is published weekly by "Billboard" magazine. It is frequently used to convey the popularity of an artist or groups of artists. Often, a recording act will be remembered by its "number ones", those of their albums that outperformed all others during at least one week. 

The chart is based mostly on sales (both at retail and digital) of albums in the United States. The weekly sales period was originally Monday to Sunday when Nielsen started tracking sales in 1991, but since July 2015, tracking week begins on Friday (to coincide with the Global Release Date of the music industry) and ends on Thursday. A new chart is published the following Tuesday with an issue post-dated to the Saturday of the following week. The chart's streaming schedule is also tracked from Friday to Thursday. 

New product is released to the American market on Fridays. Digital downloads of albums are also included in "Billboard" 200 tabulation. Albums that are not licensed for retail sale in the United States (yet purchased in the U.S. as imports) are not eligible to chart. A long-standing policy which made titles that are sold exclusively by specific retail outlets (such as Walmart and Starbucks) ineligible for charting, was reversed on November 7, 2007, and took effect in the issue dated November 17.
what is the chart based on?
what is a recording act remembered by?
how often is it published?
on what day?
are all albums included in the chart?
which are not?
can these be bought in the US anyways?
What did Nielsen start keeping track of?
when did he start?
what days were they between?
when did this change?
to what?
what happened to titles sold by retail outlets in the past?
what happened before it was reversed?
when was it reversed?
what is Billboard 200?
what is it used to convey?
is it worldwide?
are digitals eligible?
where is the list published?
the 200 most popular music albums and EPs
The "Billboard" 200 is a record chart ranking the 200 most popular music albums and EPs in the United States
by its "number ones"
a recording act will be remembered by its "number ones",
weekly
. It is published weekly by "Billboard" magazine
Saturday
A new chart is published the following Tuesday with an issue post-dated to the Saturday of the following week. 
no
. Albums that are not licensed for retail sale in the United States (yet purchased in the U.S. as imports) are not eligible to chart
Albums that are not licensed for retail sale in the United States
Albums that are not licensed for retail sale in the United States (yet purchased in the U.S. as imports) are not eligible to chart.
yes
New product is released to the American market on Fridays. Digital downloads of albums are also included in "Billboard" 200 tabulation. Albums that are not licensed for retail sale in the United States (yet purchased in the U.S. as imports) are not eligible to chart. A long-standing policy which made titles that are sold exclusively by specific retail outlets (such as Walmart and Starbucks) ineligible for charting, was reversed on November 7, 2007, and took effect in the issue dated November 17.
sales
 Nielsen started tracking sales in 1991
in 1991
Nielsen started tracking sales in 1991
originally Monday to Sunday
The weekly sales period was originally Monday to Sunday 
July 2015
since July 2015, tracking week begins on Friday (to coincide with the Global Release Date of the music industry) and ends on Thursday
Friday to Thursday.
since July 2015, tracking week begins on Friday (to coincide with the Global Release Date of the music industry) and ends on Thursday. 
it was reversed
 A long-standing policy which made titles that are sold exclusively by specific retail outlets (such as Walmart and Starbucks) ineligible for charting, was reversed 
they were ineligible for charting,
A long-standing policy which made titles that are sold exclusively by specific retail outlets (such as Walmart and Starbucks) ineligible for charting,
November 7, 2007
reversed on November 7, 2007
a record chart
The "Billboard" 200 is a record chart ranking the 200 most popular music albums and EPs 
the 200 most popular music albums and EPs in the United States
The "Billboard" 200 is a record chart ranking the 200 most popular music albums and EPs in the United States
no
The "Billboard" 200 is a record chart ranking the 200 most popular music albums and EPs in the United States.
yes
 Digital downloads of albums are also included in "Billboard" 200 tabulation
"Billboard" magazine.
 It is published weekly by "Billboard" magazine.