- Click to expand for a more detailed explanation:
Several of these series have been released in a series of five consecutively released comics using all five regular days in a week (like for instance with Secretary). This occurred several times between 2006 and 2010. But since then no series were released like this, although some series were released but with long separation between releases. That was until October 2016, when the "strange" series Time traveling Sphere were released. In the explanation for that series the strangeness of its release is discussed, relating back to how other series mentioned here have been released.
 True series
Here is a chronological list of all (22 as of April 2017) true series (removing those made for explaining certain aspects of xkcd without being a series in any clear way on xkcd):
- A five (or six) comic series released over one and a half month from September 2005. The very first comic on xkcd was named 1: Barrel - Part 1 showing that there were more than one in the series, and the next four parts (as well as 20: Ferret belonging to the series) where released within 2 months of each other. The normal schedule was not in place at that time, and the first two parts where released on the same day on LiveJournal, and they were not released in the order they are shown on xkcd today, as the order of comics where changed from that on LiveJournal when xkcd.com went live in 2006. So originally Barrel was not the first comic to be released.
- Red Spiders
- A four (or five to seven) comic series released over more than half a year from September 2005 to June 2006. The first comic was 8: Red spiders, released on the first day (as with Barrel above), but it first became a series when 43: Red Spiders 2 was released two month later with the same title and a number. A month later 47: Counter-Red Spiders came out. The last true comic in the series was 126: Red Spiders Cometh released half a year later. Only those four uses "Red Spiders" in the title. But later Red Spiders where depicted again (as in 427: Bad Timing), but those were call backs to the original series rather than as a true part of the series.
- Opening dialogue by Scott
- A three comic series released consecutive over the last part of a week. They were released the first week after xkcd comics where no longer released on LiveJournal but only released on xkcd. The first was released on January 31st 2006. The first comic released that week and only on xkcd was not part of this series. So when Randall wished to have all these three with the same title text (Opening dialogue by Scott) in the same week, the first 57: Wait For Me was released already on the Tuesday after the Monday release. This was the first time a series had been released consecutively in a week. But the three comics where not directly related.
- Parody Week
- A five comic series released over five consecutive days from Monday-Friday in August 2006. It was the first time of six that this occurred. The theme was the same, parodying other web comics, but there were no connection between the comics themselves.
- A five comic series released over five consecutive days from Monday-Friday in May 2007. It was the second series released like this, but it was the first time of four that the five comics of such a series were directly connected in a continuation story line.
- A five comic series released over five consecutive days from Monday-Friday in November 2007. It was the second series released like this where the five comics were directly connected in a continuation story line.
- A five comic series released over about five month the first 374: Journal in January 2008. It first became a series when the second comic 377: Journal 2 was released a week later at the end of January with a “2” to indicate that this was a series. Each of the first four comic was released in different weeks, the third first in April and the fourth in June. But the last comic 433: Journal 5 was released the next release day (Friday) after 432: Journal 4. This is the only series that behaves like this. Apart from that it behaves like the other five comics in one week series, with a number indicating it is a series. This thus makes it the fifth comic with five comics that are directly and chronologically ordered. (The Barrel series is in some way similar, but cannot really be considered as being the same type as this and the other four consecutive comics).
- A five comic series released over five consecutive days from Monday-Friday at the end of October 2008. It was the third series released like this where the five comics were directly connected in a continuation story line.
- A two comic series released with about two weeks between them. When the first comic 561: Well was released in March 2009 it was not a series. That was first obvious when 568: Well 2 was released in April. It is the first time that Randall made a two comic series like this by adding a 2 after the title of a previous comic. But this happened several times later, but then not always with a continuing storyline as in Well. Of course the numbering like this was also used for Journal and Red Spiders, but both of these had more comics in their series.
- The Race
- A five comic series released over five consecutive days from Monday-Friday at the beginning of May 2009. It was the fourth and last series released like this (as of 2016) where the five comics were directly connected in a continuation story line. But one more series were released like this the year after, but those comics stories where not related.
- A two comic series released with less than two weeks between them. When the first comic 595: Android Girlfriend was released in June 2009 it was not a series. That was first obvious when 600: Android Boyfriend was released later that month. It was the first series where it was only part of the title that made it clear it was a series. And no numbering was used. The plot though also makes the connection clear. A similar connection between comics in series was not seen before Time traveling Sphere seven years later (see below).
- Online Communities
- A two comic series released with more than three years between the first 256: Online Communities in May 2007, and the second 802: Online Communities 2 from October 2010. It was first the latter that with the same title and a number 2 made it into a series. This is the reason it is first listed chronologically here. It was not a series in 2007. Adding the number “2” to make it into a series is like the Well series, and this was the second time that happened.
- Five-minute comics
- A three comic series released consecutively over a week using the normal release schedule in the middle of November 2010. This is the second time a series has been released with three in one week, following the Opening dialogue by Scott in 2006 (but that time there were four comics in the week, one not belonging to the series). It is thus the only time that a week with three comics only where all released as a series. It was the first of two weeks in a row with series. Both series were made to give Randall some time to cope with his wife’s cancer.
- Guest Week
- A five comic series released over five consecutive days from Monday-Friday at the end of November 2010. It is so far (as of 2016) the last time of six that this occurred. The theme was the same, with five web comic artist making guest appearances on xkcd, so there were no connections between the comics themselves. It is the second time this happened, the first time was the first five day series in August 2006. It was the second of two weeks in a row with series. Both series were made to give Randall some time to cope with his wife’s cancer. Since this comic there has never been released more than three comics in a week. Also until October 2016 with the Time traveling Sphere there were never to comics directly related which were released in the same week (see below).
- xkcd Phones
- A six comic series (as of 2017) released over more than three years between the first 1363: xkcd Phone in May 2014 and the latest 1889: xkcd Phone 6 from September 2017. It was first with the release of the second 1465: xkcd Phone 2 in December 2014 with the same title and a number 2 that it became a series. Adding a number to a title has been used like this two times before, but it is the first of this kind of series to progress beyond 2, so far all the way to 6. Only in Journal (that went to 5) did the numbering without including "part" progress beyond 2, and that was a true series where the story was connected. This is also what makes this a special type of series in xkcd not seen before, as it is clear that these are a series, but they are not connected as a story.
- Exoplanet Names
- A two comic series released with almost a year between the first 1253: Exoplanet Names in August 2013, and the second 1555: Exoplanet Names 2 from July 2014. It was first the latter that with the same title and a number 2 made it into a series. This is the reason it is first listed chronologically here. It was not a series in 2013. Adding the number “2” to make it into a series is like the Well series, and this was the fourth time that happened and so far (as of 2016) the last time. These two comics are not connected as a story, rather the second is an update to the list made in the first, even including most of the first list. In that was this is a special type of series, in someway related to the xkcd phone series above. The first of these two comics came out before the first phone comic, but the second phone comic came before the second of these, thus explaining the chronology.
- Synonym Movies
- A two comic series released with less than two weeks between them. When the first comic 1563: Synonym Movies was released in August 2015 it was not a series. That was first obvious when 1568: Synonym Movies 2 was released later that month. There is no storyline connection between the two, but the idea is the same, showing DVD boxes with real movie titles changed using synonyms.
- Substitution series
- A three comic series (as of 2016) released over almost three years between the first 1288: Substitutions in November 2013 to the third 1679: Substitutions 3 from May 2016, It first became a series with the second comic 1625: Substitutions 2 released in January 2016, explaining why it is placed after the two above. It is not a continuing series, but rather the same concept used again, more related to the xkcd Phone or Exoplanet names above, than to a comic series like Well. It is the only series so far released like this over long time that is at three releases, as xkcd Phone is now t four.
- Code Quality
- A two comic series (so far as of 2016) released with more than a year between them. When the first comic 1513: Code Quality was released in April 2015 it was not a series. That was first obvious when 1695: Code Quality 2 was released in June 2016. This series reminds of Well and Android, but with much longer time between the two released.
- Time traveling Sphere
- A two comic series released over two consecutive but normal release day in the same week (a Monday and a Wednesday) in the middle of October 2016. This was a very strange release in many ways, and that has been described in great detail in the series category. It was first clear that this was a series with the Wednesday release 1748: Future Archaeology but not from the title which has very little direct relation to the first title 1747: Spider Paleontology, but the plot was clearly a continuation of the first comic. The series could in principle continue later, but it did not fill out the entire week, and seems to have stopped at two (as of only two more releases after the second of these came out).
- Bad Map Projections
- So far two comics have been released but given they have numbers 79 and 108 there could be many more to come.
- The first was released on January 11th 2017, it was #107: 1784: Bad Map Projection: Liquid Resize. And it became a series with the second comic released just a little more than a month later on January 15th with #79: 1799: Bad Map Projection: Time Zones.
- ISS Solar Transit
- Two comics was released Monday and Friday in he same week. Just at with the Time Traveling Sphere mentioned above, this was yet a new way to release a series.
- The first comic, 1828: ISS Solar Transit, was released on April 24, 2017 and the second, 1830: ISS Solar Transit 2, four days later on April 28, 2017. Once again a series where the second comic just adds a "2" to the title.
 Other series
Here is a list of the (2 as of 2016) other types of series that has been used for explaining certain aspects of xkcd without being a series in any clear way on xkcd:
- A Smarter Planet
- A two comic series from the summer of 2009 made specially for IBM's blog A Smarter Planet, and thus not part of the regular xkcd comics. Although the obviously have been made as a set, their stories are not related in any way, and their titles Conservation and Prescriptions are also not related. So they would only make a series because they were made for the same specific reason.
- Comics sharing name
- This category lists comics that share name with another comic, or almost shares name. So many series would be here. But there are also some series like comics that are not directly related in a series, like the new years comics released under the title of the new year. (2012, 2014 and 2016 so far). Many of the comics that share name or almost share name is not in any way part of the same series. The category is mainly used for explaining comics with similar name.
This category has the following 24 subcategories, out of 24 total.