the Undergirls (アンダーガールズ, or Under Girls) are an special group/team of girls that since the term was included in the 1st Senbatsu Election, take charge of the main c/w (B-side) of most of the AKB48 Singles and recently also SKE48 and SDN48 Singles.

History

the Undergirls were introduced in the 1st Senbatsu Election, where the girls that ranked from 22# until 30# were given the chance to sing the b-side track of the single (Iiwake Maybe) that the senbatsu elected (21#~1#) were going to sing.

previous to this, all the original b-sides were also done by the Senbatsu of that single, what now gave the chance to a lot of other girls to actually take part of a single release (with most of the them doing it for the first time).

the first Undergirls generation, consisted of 9 girls and sung the track Tobenai Agehachou, which also got a video. marking it as the first time an AKB48 c/w got a Music Clip.

later on for the next single RIVER (that also introduced the Theater Girls), the Undergirls concept was used again. with the difference that this time, the manager decided the members making an almost completely different selection of girls compared to the first generation.

the Undergirls weren't used in the following single (Sakura no Shiori) and made a comeback in Ponytail to Shushu. since then, the Undergirls and were used on a streak until Beginner, giving the total of "Undergirls songs" to 5.

Undergirls on SKE48

for SKE48's 3rd Single Gomen ne, SUMMER, the Undergirls concept was implemented with SKE for the first time.

in difference with AKB, 2 different groups of Undergirls were used in the same single ("Undergirls A" and "Undergirls B"). both groups featured an small group of girls and different songs Undergirls A was an Exclusive-track and Music Video of Type-A as Undergirls B were of Type-B.

Undergirls on SDN48

SDN48 started using the Undergirls concept since the their first single, also implementing the Undergirls A/Undergirls B concept SKE48 started.

"Undergirls" & "Theater Girls" as Popularity level used by the Fandom

The Undergirls are usually given most of the spotlight after the Senbatsu in a single release, making the other Single-exclusive group (the Theater Girls) feel as the "least important" group of girls for the manager.

the Theater Girls were introduced in RIVER, a single that was promoted as "first single ever where all the 48 girls of the group take part". with Senbatsu still being "the most important" selection of girls, the Undergirls were used once again and a new group named the Theater Girls was introduced. (see full article: Theater Girls)

in difference with the Undergirls, the Theater Girls didn't get a new original song and got to cover an AKB48 Stage Song instead. -as well as getting their pv filmed in the AKB48 Theater as a "performance music clip", instead of an "original PV" like the Undergirls and Senbatsu get. (what makes the fans that "the Theater Girls don't get as much budget as the Undergirls")

not only for this, the Fandom sometimes notice how some girls are pushed more than other from the manager. and if those girls aren't popular enough, they mostly end up taking part of Undergirls instead of Theater Girls.

since the Type-A and Type-B releases were included in AKB48 Singles, the Theather Girls started to take part of only one of the editions, compared to the Undergirls that appear in both as the main bside of the single.

other difference between both groups is that the Undergirls are more of an actual regular subgroup, while the Theater girls don't necessarily appear in all the singles (specially in those where the Senbatsu Elections are involved)

in the AKB fandom, you will normally see fans classificating the girls as "Senbatsu" "Undergirl" or "Theater Girl" , referring actually more to popularity-level instead of how much they are pushed by the manager. (always depending on the context, of course)

Undergirls Generations (AKB48)