There are, effectively, five significant “currents” in Mainstream AC at this or any given moment. Of the current top five, only one, Ed Sheeran’s “Perfect,” is still considered a current in any other contemporary format. The others — Pink, “What About Us”; Shawn Mendes, “There’s Nothing Holdin’ Me Back”; Chainsmokers & Coldplay’s “Something Just Like This”; and Sheeran’s “Shape of You” — have long moved to recurrent at Hot AC and Mainstream Top 40.

“Shape of You,” released in January 2017, has spent 58 weeks on the AC chart. Mendes was a CHR hit last summer. The Chainsmokers/Coldplay song peaked at CHR last May. Charlie Puth’s “Attention” peaked at CHR relatively recently — last September. But it got my attention, and spurred this column topic, when it was one of the top 5 “most added” at Mainstream AC two weeks ago, even as the next single, “How Long,” sits in the top 5 at CHR. “How Long” is currently No. 17 at AC — meaning about 125 spins outside overnights — but will undoubtedly be top 5 itself, probably in late spring.

Of the songs in the top five, three have at least one follow-up in play at other formats. “Shape of You” has had two follow-ups, one of which is “Perfect.” Only “Perfect” and “There’s Nothing Holdin’ Me Back” (which was a second-edition add-on to an existing album project) are not “the previous single” somewhere.

Major AC stations such as WBEB (101.1 More FM) Philadelphia, WMJX (Magic 106.7) Boston, and KQXT (Q101.9) San Antonio, Texas, are exactly aligned with the chart top 5. Others differ mostly because some of their top five or so songs are as old or older: “Water Under the Bridge,” “Scars to Your Beautiful.” Of the reporting ACs in PPM markets, only one, KSNE (Sunny 106.5) Las Vegas, shares only one power current with the chart.

The average number of currents played by the 35 AC chart reporters in the PPM-measured markets is eight. That number was determined by counting songs in the station’s most-played rotation, whatever they may be, plus adding those songs that were still developing on the AC or other charts. A few stations like WLTW (Lite FM) New York seem to have a secondary rotation for currents; more typically, the second-most-spun rotation at Mainstream ACs is a mix of stay-currents, recurrents, and 1-2 developing titles. Because it’s a current elsewhere, I counted Mendes as a current for WDOK Cleveland, even as it was backed off into a category that also includes more obvious recurrents such as “Can’t Feel My Face” and “Ex’s and Oh’s.”

A few years ago, at the height of the mother/daughter excitement about current hit music and the CHR format, a few stations seemed to be pushing past the accepted template of one “current” and one recurrent an hour. WALK Long Island, N.Y., kept going and evolved to Hot AC. So did KYXY San Diego, which foundered there, and regrouped to a more traditional AC. Now, only WLIF (Today’s 101.9) Baltimore, pushes the envelope on tempo and texture. Its 13 currents do include “How Long” in power, for instance.

Is the current AC template a surprise? Not to any AC programmer or anybody who promotes to the format. Justin Timberlake’s “Can’t Stop the Feeling,” 2016’s summer CHR smash, was No. 1 at AC the following year. (“I knew when I heard it that it would be the Song of Winter,” I tweeted at the time.) But it feels like it has been exacerbated due to other paradigm changes at AC and a general lack of consensus hits at CHR.

Is it a problem? Am I telling WLTW (Lite FM) New York that more than nine currrents would improve its market-leading 8.5 share (even after Christmas music ended)? KOST Los Angeles, with five currents, was only a 5.5 after holiday music, but that still makes it No. 2 in the market, ahead of CHR KIIS.  But not every Mainstream AC is Lite FM or KOST. And WALK is leading its market as well. For that matter, WLTW was successful in the mid-‘00s as well, when it helped bring songs like “Bad Day” and “You’re Beautiful” to American radio.

So how did we get here? In the ‘80s, stations like WLTW were built on having no real currents — perhaps a song from the last few years might test in as a result of research once or twice a year. About 20 years ago, stations like KOST, which once played a full boat of currents, went down to a then-radical single rotation of nine titles. Eventually those models converged.

In the early 2000s, when I was still involved with radio industry charts, I argued unsuccessfully that stations like KOIT San Francisco that played about six recent songs should become part of those charts. Wouldn’t it be valuable to know what those six songs were, I contended? At that time, there was little support for that argument. Now, you couldn’t have a Mainstream AC chart if you excluded stations with five recent titles.

“Adults know what they like and like what they know” has long been AC’s key programming tenet. For a while, however, it looked like CHR’s mother/daughter coalition could change that. The Pink/Katy Perry/Kelly Clarkson era certainly changed the tempo and tenor of what Mainstream AC played. With CHR developing such a quorum among 40-year-old-women, I wondered if the “like what they know” part might be happening sooner. But only a few stations like KYXY and WALK got close to finding out.

With CHR serving as such a reliable feeding ground for Hot AC and Mainstream AC, those formats began doing less to develop their own hits, even seemingly obvious ones. Ballads that aren’t “Perfect” have had a harder time developing traction at AC. Even Sam Smith’s “Too Good at Goodbyes” never made that consensus top 5 at AC, and actually peaked higher at CHR (No. 6) than AC (No. 7).

Over the last two years, the tenor of CHR hits has changed. The artists that helped make Mainstream AC more millennial —  Perry, Pink, Clarkson, now even Taylor Swift — had a lesser CHR footprint. The decline of shared mother/daughter music has left adults less likely to hear something they like on Top 40 and kids more likely to be looking elsewhere. That hasn’t been enough to eliminate CHR as AC’s gatekeeper. But it has further limited what might be coming through the pipeline.

Again, is this a problem? Maybe a missed opportunity. The CHR Miracle of the late ‘00s/early ‘10s proved that adults were sincere in wanting to hear new music, if only they could find the new music they liked. It also shattered the assumption that music would be somehow esoteric. It might be Adele. It might also be Pitbull or “Party Rock Anthem” for a minute. Now adults are again looking for new music they might like; the difference is that in the intervening decade, it’s become easier for anybody to find music for themselves.