Here are two examples of the importance of a single chord.
First, The Beatles' "A Hard Day's Night"
The title came from one of Ringo's unique utterances:
The song's title originated from something said by Ringo Starr, the Beatles' drummer. Starr described it this way in an interview with disc jockey Dave Hull in 1964: "We went to do a job, and we'd worked all day and we happened to work all night. I came up still thinking it was day I suppose, and I said, 'It's been a hard day...' and I looked around and saw it was dark so I said, '...night!' So we came to 'A Hard Day's Night."
George Martin describes the origin of the chord:
We knew it would open both the film and the soundtrack LP, so we wanted a particularly strong and effective beginning. The strident guitar chord was the perfect launch.
Musicians from then to this very day, have been trying to reproduce the chord, figuring they could do it with a single guitar.
There have been a number of theories as to the identity of the chord. Over the years, suggestions have included the following:
A dominant 9th of F in the key of C
G-C-F-Bb-D-G
C-Bb-D-F-G-C in the key of C
A polytriad ii7/V in Ab major
G7sus4 (open position)
D7sus4 (open position)
G7 with added 9th and suspended 4th
A superimposition of Dm, F, and G
Gsus4/D
G11sus4
G7sus7/A
Dm11 with no 9th
Gm7add11
G9sus4/D
The chord was confirmed by George Harrison as an Fadd9 during an online chat on 15 February 2001:
Q: Mr Harrison, what is the opening chord you used for A Hard Day’s Night?
A: It is F with a G on top (on the 12-string), but you’ll have to ask Paul about the bass note to get the proper story.
Giles Martin tells Randy Bachman how to do it:
So, end of story, right?
Nope.
It's not even just one guitar. It's two guitars (one of them 12-string), a piano and a bass!!:
Next, we have one of the most discussed chords in all of classical music.
"The Tristan Chord"
Named this, because it is the defining first chord of Richard Wagner's opera "Tristan und Isolde" (completed 1859)
Wagner described the opera as “one of endless yearning, longing, the bliss and wretchedness of love; world, power, fame, honor, chivalry, loyalty and friendship all blown away like an insubstantial dream,” for which there is “one sole redemption — death, finality, a sleep without awakening.”
Woody Allen described Wagner this way:
“I just can't listen to any more Wagner, you know...I'm starting to get the urge to conquer Poland.”
― Woody Allen
The chord itself was not new, but it WAS new the way Wagner used it.
It begins the Prelude. While not the first sound made, it is the first chord. It begins at 8 seconds into this video:
The Tristan chord is a chord made up of the notes F, B, D♯ and G♯. More generally, it can be any chord that consists of these same intervals: augmented fourth, augmented sixth, and augmented ninth above a bass note. It is so named as it is heard in the opening phrase of Richard Wagner's opera Tristan und Isolde as part of the leitmotif relating to Tristan.
Here is actor/comedian Stephen Fry talking to German pianist Stefan Mickisch about it:
Maestro Asher Fisch: "What E=mc2 did to physics, the Tristan chord did to music in the 19th century."
This music is used to great effect in the Lars von Trier film "Melancholia"