TLDR: Idol Song is essentially J-Pop sung by Idols instead of J-Pop singers. There are no requirements for a song to be classified as Idol Song, but there are common conventions that one can expect when they listen to Idol Song.

Idol Song (アイドルソング) is the name of one of the pseudo-genres from Japan surrounding Nerd Culture. Idol Song is the songs that idols sing. The most popular comparison is something called Anisong (アニソン). Anisong is songs that are in Anime (mostly OPs and EDs). Another psudo-genre is Game Music (ゲームミュージック) Game Music is music that appears as Background Music in Video games.

I call these genres pseudo-genres“ because unlike a normal genre that have uniting conventions like BPM ranges, instrumentation, song forms, common chord progressions, etc., these pseudo-genres have no uniting conventions. They become the pseudo-genre merely by appearing in the type of media. Game Music is Game Music if I was placed in a Video Game, period. It doesn't have to be chip-tunes, nor classical, nor looping, nor rap, not catchy, not midi, not live. If a developer put a song in a video game it becomes Game Music, period. Likewise, no matter the musical genre, the lyrical content, or the instrumentation, Idol Song becomes Idol Song if a person identifying as an Idol sings the song. This is not to say, as we will discuss later, that there are not cultural expectations around what Idol Song is, that can either be played straight or subverted.

In general, Idol Song is just J-Pop sung by Idols. So to understand Idol Song is to understand J-Pop. The more you study J-Pop, the easier it is to recognize the cultural expectations of Idol Song that likely will be present in any given song you hear by your favorite Idol. All Japanese Nerd genres are built off of J-Pop, so understanding J-Pop, again, will help you understand Game Music or Anisong. So most everything I talk about will be in reference to J-Pop.

Feel free to check out this guide to Making Idol Song.

Let's start with structure first. There are two main flavors of songs in J-Pop. The Full Chorus, and the 1 Chorus.

A Full Chorus is a structure that we are used to in America, its similar to the radio format. Under 5 minutes (like 7in shellac singles), multiple choruses, and very catchy (repeating words over and over). The basic Full Chorus starts with an Intro (イントロ), has an A melody (Aメロ), a B melody (Bメロ), a C Melody (often called the Sabi [サビ]), a D melody (a bridge), and an Outro (アウトロ). Generally all the parts will be the same amount of Bars in length, typically 8. Some times an instrument solo (ソロ) is thrown in around the D melody for a little extra spice.

A One Chorus usually will be very short and have the Intro, to the A melody, to the B melody, then end with the C melody (There was only 1 C melody, as opposed to a normal full chorus song that would play the C melody 3 times). A Full Chorus usually is just literally 2 One Choruses plus a D Melody and then the C Melody 2 more times, and then there is an Outro which is the same, or similar to the intro (which may have been played twice already). During a Full Chorus, the Intro, Kansou (間奏) (when they play the Intro after the 1st Sabi), and Outro will have different names in the song structure, but will likely sound identical or really similar. If the fans are going to call Mix, they probably are going to do it during the intro sections because they Idols are not singing there. I personally am not in a fandom that calls Mix, so I know very little about it.

So there is not one single chord progression (コード進行[KooDo Shinkou]) that explains all of J-Pop. J-Pop is a mix of a whole bunch of different styles, genres, and Music Theories. Essentially J-Pop is the child of Kayoukyoku and is merely a continuation of the style by incorporating more Western sounds. I think of Kayoukyouku as Soul Music and J-Pop as 90's RnB. I mean, functionally they are the same music, but one is analog and the other is digital, one is less sophisticated, the other a bit more sophisticated. So just think of Kayou as the Old School, and J-Pop the New School. Its the same music, just another generation.

So J-Pop has influences from Western genres, but it is not merely singing Western Songs in English. The Japanese really merged American genres in a way that Americans did not because of our legacy of Jim Crow Segregation (by the way our American music industry remains segregated even today, but is getting better because of the internet and the Music Industry's weakening grip on the gates of entry). Jazz music theory permeates J-Pop. But also Japanese musicians can not un-hear their own traditional music, so that creeps in from time to time (Manifesting itself in things like having a song with a chord progressions based on western music theory, but singing the Sabi of that song in a Japanese Yo Scale).

However, like most music based on the blues, the Chord Changes will inevitably boil down to 1-4-5. The Japanese however, favor Jazz changes and will take forever to get through that 1-4-5. This makes a lot of J-Pop sound sophisticated (because it is). J-Pop is fundamentally Western Music, its Roots lie in America and Europe, and as such the instrumentation likewise is mostly based on Western Instruments (guitars, basses, drum sets etc…)

I have no idea where, but I once heard someone say that the Idol Industry is “A little Embarrassing.” Its embarrassing for the Idols, wearing those costumes and singing crazy songs to middle aged men. And its also a little embarrassing for a middle aged man to support this Idol he has no chance with by giving her enough money to start a college fund… but I digress.

Idol Song is normally: sung by cute young women; involves lyrics concerning puppy love, unrequited love, having fun, or other youthful centered activities ; cute instrumentation; upbeat (over 140 BPM); musically unsophisticated (4/4 time, simple harmonization, little to no key changes, etc…; and sung in unison or with simple voicing. If you hear a song that checks all these boxes it is almost certainly Idol Song. There is this one thing the drums do that I have no idea what its called, but I know its a holdover from Kayou. Often times a song will signify a “feel change” by changing the snare hit from 2 and 4, to soley on 3. Since most J-Pop no longer uses this switch often, its usually a marker that you are listening to Idol Song. The “feel change” is a marker to fans to change the rhythm of their chants.

Further more there are some melody specific nuances that have become pretty common place in Idol Song. The melody will frequently leave empty bars or sections so that fans can cheer in the empty spots. Often times it will be something like a bar of melody and a bar of silence or 14 beats of melody and 2 beats of silence. Non- ballad Idol Song melodies are specifically crafted with fanchanting in mind. So another dead give away is whether you could chant a 2 or 3 syllable phrase after the Idol stops singing a line. But this is not exclusive to Idol Song as Enka's melodies are written the same way (but fans may still be chanting tho, even in Enka).

Bubblegum KK (Animal Crossing)

This song ticks a lot of idol boxes as its a “parody” of the genre. Note the melody, not the “feel change”, note the instrumentation.

Parodies often embody alot of the conventions of

Fan Club (Rhythm Heaven)

Note the instrumentation and the melody.

Fan Club Live (sung by Erina)

Psycho Solider (Kaori Shimizu) (1987)

Note the simple arrangement and the melody.

The Idolm@ster (2005)

Pay attention to the melody and how it fits in with the fanchants. There is a lot of space within the melody and the melody is very rhythmically consistent.