It was 50 years ago that Led Zeppelin launched.

There are a number of days that could claim that distinction, their first rehearsal was in the late summer of 1968, while their first show together was Sept. 7, 1968 as The New Yardbirds, fulfilling the obligations that were booked for a Scandinavian tour by then-Yardbirds manager Peter Grant before the original band broke up. Zeppelin’s first album was recorded in October and November, and the latter month also saw the announcement that their first album would come out on Atlantic in early January. They launched their first American tour in Denver on Dec. 26, and the first album, Led Zeppelin I, came out in January.

In fact, the date of their debut is usually cited as Jan. 12, but that was a Sunday in 1969, and blue laws -- in which some commerce and leisure activities were restricted on Sundays on religious grounds -- pretty much ruled out Sunday retail in the U.S. then. More likely, the release was set for the week of Jan. 12., with the album trickling into stores across the country on whatever days it arrived in the mail.

It’s well known by now how the band came together and got its name from the recording session for “Beck’s Bolero” in 1966 for Jeff Beck, with Jimmy Page, Beck, John Paul Jones, Nicky Hopkins and The Who’s Keith Moon, with the latter joking that they should form a band which would probably go over like a lead balloon.

It was Page who brought Jones aboard; the two knew each other from countless sessions backing other bands, including the recordings for Herman’s Hermits which, legend has it, Page often played on; while Jones also often served as the music director for Hermits recording sessions for producer Mickey Most, who also produced the Yardbirds.

Meanwhile, one of the singers Page wanted for his band, Terry Reid, suggested Robert Plant, who’d been trying to launch a solo career when not singing with Alexis Korner. Plant knew drummer John Bonham from the Band of Joy, and when they got together with Page and Jones, magic happened; and it kept on happening right up until the day the band broke up, due to Bonham’s death in 1980.

To coincide with the band’s and LZ 1’s 50th anniversary, as well as Page’s 75th birthday today (Jan. 9), Billboard has ranked the Led Zeppelin catalog, song by song, based on revenue generated by digital activity since the band’s music first became available at download stores like iTunes and on-demand services like Spotify. (The methodology on how this ranking was compiled can be found at the bottom of this post.)

But it's important to note that while revenue serves as the basis of the ranking, Billboard only counted revenue where the consumer made a choice, either by downloading a song or playing a stream on-demand. That means that other revenue generators -- where professionals make choices like which songs to play on the radio, or which song to put in a TV show or to physically release as a single -- were not considered, because the consumer wasn’t involved in those choices.

Therefore, this list is a true gauge of the popularity of the songs within the band’s catalog, because the Zeppelin fans’ actions generated the revenue upon which this ranking is based. Here now is Billboard’s ranking of the most popular Led Zeppelin songs as voted on by U.S. consumers with their time and money, with an observation or two about each of the 94 songs -- total revenue generated: $21,607,542.71 -- in the band’s catalog.

1. "Stairway To Heaven"
Total Revenue: $2,903,223.42

It's no surprise that this is the top revenue generator for Led Zeppelin, as it's without a doubt the band’s most well-known song. While for years it might have received too much airplay on FM radio, nowadays its plays are way down -- about 22,000 spins so far this year, versus a song like Ed Sheeran’s “Shape of You,” with 739,000 spins year-to-date, according to Nielsen Music -- which means that it's a pleasure when it comes on the air. Spirit claimed its song “Taurus” was infringed by "Stairway," but a court ruled in Zep’s favor in June 2016, although an appeals court partially vacated that decision due to improper jury instructions and remanded the case for a new trial. While the acoustic melody in the early passages of "Stairway" is slightly reminiscent of "Taurus," when Zep's opus goes full-blown electric and grows into other melodic stages, the resemblance is naught. If Zeppelin did borrow its acoustic part from the same place that "Taurus" gets its guitar melody -- as was argued during the first trial -- Zeppelin’s use of it in "Stairway" certainly seems transformative. (Also, "Taurus" is a completely forgettable song, while "Stairway" has already proved itself to be a song for the ages.)

Early on, almost a year before the song came out, there was a small story in Circus magazine that hinted at the song, talking about how the song was going to build as the melody grows and could be as long as 15 minutes. Around the same time, Page and Plant were talking about creating a song that would live forever like Beethoven’s 5th, a hint of what was coming. Certainly, "Stairway," on the band's most popular album, IV (aka ZOSO), is their best shot at that.

2. "Kashmir"
Total Revenue: $1,421,130.32

From the Physical Graffiti album, this is the song that Plant wishes was the band’s most popular song instead of “Stairway.” While it is an otherworldly, epic song, instilled with rhythms and melody patterns from the Middle and Far East, some Zeppelin aficionados will argue that a few other Zep epic numbers surpass this song.

3. "Immigrant Song"
Total Revenue: $1,306,140.94

Although always a favorite, it nevertheless turns in a surprise showing in appearing so high in the rankings. It's the lead track and single from the Led Zeppelin III album, one of Zeppelin’s less commercially-successful albums -- only six times platinum -- but one that nevertheless is among the most appreciated albums by the Zep faithful***. While garnering a fair amount of radio play down through the years -- and used as a set opener on early tours -- it returned to the mainstream again when it was used as a major centerpiece in the 2017 movie Thor: Ragnarok.

4. "Black Dog"
Total Revenue: $1,167,232.19

Another surprise for appearing so high on the list, "Black Dog" was released as a single off IV and broke into the top 20 of the Billboard Hot 100 at No. 15, but certainly wasn’t a hit by any stretch of the imagination, i.e. the oldie stations don’t play it when they’re playing music from 1971. (Though the classic rock stations certainly do.) The song’s main riff was composed by John Paul Jones, and with its shifting time signatures, off-beat drum pattern and the riff folding almost back in on itself, musicians say it's a hard song to cover. Still, it's got one honey of a guitar line and a good vocal melody, too. Of all the songs Zeppelin is known for, this is one of those paramount in owing its livelihood to electricity. Jones would revisit "Black Dog" for inspiration on “Crackback," a song with Page on guitar off the Scream For Help Soundtrack album that he composed and recorded after Zeppelin.

5. "Whole Lotta Love"
Total Revenue: $1,034,129.29

Zeppelin’s second-most well-known song, even if it's only the fifth-most popular one, made it all the way up to No. 4 in the Billboard Hot 100 back in 1969, becoming the band's highest-charting song. Zeppelin has a reputation for not being a singles band; legend has it that they didn’t release any singles at all. but that was only true in the U.K. They released plenty of singles in the U.S. from the get-go -- at least one from each album. (But they never put “Stairway To Heaven” out as a single, helping that album to sell like a hit song.)  “Whole Lotta Love” is the song that Plant once feared he would be forced to sing when he was old and in a wheelchair. While Plant seems to dislike the heavier side of Zeppelin nowadays -- and this song appears to be at the heart of what he is trying to avoid by not doing a Zeppelin reunion -- it's still one he plays with the Shape Shifters, and he also does a pretty faithful rendition.

6. "Ramble On"
Total Revenue: $888,793.61

Another surprise in being up so high in the rankings. If Zeppelin ever deigned to release a fully acoustic album, this would be on it. Like more than a few Zep tunes, it blends sparse acoustic passages that explode electrically into hard rock motifs.

7. "Over the Hills And Far Away"
Total Revenue: $757,125.57

This was the B-side of a single from Houses Of the Holy and shares the soft-LOUD dynamics displayed in the previous song. The acoustic intro tunings are borrowed from Page’s "White Summer," a song from his Yardbirds days; and which Page apparently borrowed then from Davy Graham’s interpretation of the traditional Irish folk song “She Moves Through The Fair.”

But this time around, the guitar riff upon which the song is built loses the jagged-edged, raga-sounding tuning and is transformed into crystal-clear, prettified Western chords to produce a nice acoustic melody -- that is, until Page unleashes an in-your-face rock tsunami. While some say this tune has one of Page's least-distinct guitar solos, they might be missing the point: He seems to be concentrating more on experimenting with different guitar tones than worrying about what notes to play, a tactic that would be more on display on later albums.

8. "Goin' To California"
Total Revenue: $694,689.56

Zep’s ode to Joni Mitchell. Certainly one of the Blimp’s prettiest songs, it was a staple in the live set for many a tour.

9. "Rock n’ Roll"
Total Revenue: $636,985.97

It was supposedly created out of a jam between Page and Bonham, where the lads knew immediately they had struck a big fat rock progression, so they rolled back the tape to hear that they had created a unique repackaging of a classic rock n' roll riffs. Sometimes used as a set opener; always a guaranteed rocker.

10. "D'yer Mak'er"
Total Revenue: $553,459.73

A joke song lyrically, this shows the band stepping way beyond the musical bounds they had established on their first four albums. It was the first song played by radio ahead of the Houses Of The Holy album and is remembered by many for initially being a big disappointment -- the Zep faithful weren’t prepared for this new side of Zeppelin. But when radio started playing the rest of the album, fans reacted accordingly and the album became a big seller, and with hindsight this song grew on Zep fans. “Houses Of The Holy” was at the heart of the tour that would establish the group as the biggest band in the world to the mainstream. Before, only music fans and the industry executives were aware of how successful they were.

11. “When the Levee Breaks”
Total Revenue: $547,514.60

This song from IV, along with “No Quarter” from Houses Of The Holy, finds Zeppelin stretching the traditional blues idioms into new metallic directions. The drumming sound achieved by Bonham and captured by producer Page on this track has been known to be sampled on many another recording.

12. “All My Love”
Total Revenue: $536,716.87

Appearing on the band's last studio album, 1979’s In Through The Out Door, Robert Plant's tribute to his son who died tragically at an early age was written with John Paul Jones, who brings his multi-instrumental prowess to the fore in turning in one of Zeppelin's prettiest songs. That’s a phrase that most would think an oxymoron, but for the existence of this song and its competitors for that title: "Going To California," "The Rain Song," “Ten Years Gone” and "Tangerine," among others.

13. “Fool In The Rain”
Total Revenue: $497,152.94

El Zep takes a left turn yet again surprising fans by blending various South American rhythms and melodies with Zeppelin's own inimitable sound. Found on the band’s final album, In Through The Out Door, where Page's touch is at its lightest, it showcases the other three members, particularly Jones and Bonham, until the latter practically overtakes the track in the accelerated middle section.

14. “Good Times, Bad Times”
Total Revenue: $484,799.06

One of Zeppelin's finest moments in the view of the Zep aficionados, “Good Times, Bad Times” is the first song on the band’s first album -- and what a fine way to debut the band. At less than three minutes, the song showcases Zeppelin at their finest hard-rocking selves.

15. “Dazed And Confused”
Total Revenue: $443,730.10

The first Led Zeppelin classic opus is also on the first album and became a highlight of their live shows. It’s also one of two songs -- “How Many More Times” being the other -- where Page takes out his violin bow and pulls out all of his showman stops. On the recording, while Zeppelin's playing and arrangements make the song their own, the melody, some of the lyrics and the use of dynamics and long instrumental passages were lifted from “Dazed and Confused” by Jake Holmes, who was uncredited on the album until an out-of-court settlement in 2011 set things right. (Holmes’ original version is highly inventive and amazing as well; all Zeppelin fans and even haters should check it out.) As for Zeppelin's version, in addition to what they borrowed from Holmes, Page re-channels the guitar solo from "Think About It," a song he wrote and played with the Yardbirds -- it was the last track they ever recorded, according to legend -- and places it at a key juncture in “Dazed and Confused,” making it one of his most well-known, and best, guitar solos.