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HTML 

              
                <form class="synth chord" id="app">
  <h2>CHORD VISUALIZER</h2>
  <fieldset>
    <label><strong>Key:</strong><select id="key"><option value="A">A</option><option value="A#">A#</option><option value="B">B</option><option value="C">C</option><option value="C#">C#</option><option value="D">D</option><option value="D#">D#</option><option value="E">E</option><option value="F">F</option><option value="F#">F#</option><option value="G">G</option><option value="G#">G#</option></select></label>
    <label><strong>Chord:</strong><select id="chord"><option selected="" disabled="">Select Chord</option><option value="Major triad">Major triad</option><option value="Minor triad">Minor triad</option><option value="Augmented triad">Augmented triad</option><option value="Diminished  triad">Diminished  triad</option><option value="Dominanth seventh">Dominanth seventh</option><option value="Major seventh">Major seventh</option><option value="Minor-major seventh">Minor-major seventh</option><option value="Minor seventh">Minor seventh</option><option value="Augmented-major seventh">Augmented-major seventh</option><option value="Augmented seventh">Augmented seventh</option><option value="Half-diminished seventh">Half-diminished seventh</option><option value="Diminished seventh">Diminished seventh</option><option value="Dominant seventh flat five">Dominant seventh flat five</option><option value="Major ninth">Major ninth</option><option value="Dominant ninth">Dominant ninth</option><option value="Dominant minor ninth">Dominant minor ninth</option><option value="Minor-major ninth">Minor-major ninth</option><option value="Minor ninth">Minor ninth</option><option value="Augmented major ninth">Augmented major ninth</option><option value="Augmented dominant ninth">Augmented dominant ninth</option><option value="Half-diminished ninth">Half-diminished ninth</option><option value="Half-diminished minor ninth">Half-diminished minor ninth</option><option value="Diminished ninth">Diminished ninth</option><option value="Diminished minor ninth">Diminished minor ninth</option></select></label>
  </fieldset>
  <div id="kb49" class="kb kb--49"><button aria-label="C3" data-freq="130.8127826502993" name="midi_48" "="" style="--gcs:1" type="button>"></button>
<button aria-label="C#3" data-freq="138.59131548843604" name="midi_49" "="" style="--gcs:3" type="button>"></button>
<button aria-label="D3" data-freq="146.8323839587038" name="midi_50" "="" style="--gcs:4" type="button>"></button>
<button aria-label="D#3" data-freq="155.56349186104043" name="midi_51" "="" style="--gcs:6" type="button>"></button>
<button aria-label="E3" data-freq="164.81377845643496" name="midi_52" "="" style="--gcs:7" type="button>"></button>
<button aria-label="F3" data-freq="174.61411571650194" name="midi_53" "="" style="--gcs:10" type="button>"></button>
<button aria-label="F#3" data-freq="184.99721135581723" name="midi_54" "="" style="--gcs:12" type="button>"></button>
<button aria-label="G3" data-freq="195.99771799087463" name="midi_55" "="" style="--gcs:13" type="button>"></button>
<button aria-label="G#3" data-freq="207.65234878997256" name="midi_56" "="" style="--gcs:15" type="button>"></button>
<button aria-label="A3" data-freq="220" name="midi_57" "="" style="--gcs:16" type="button>"></button>
<button aria-label="A#3" data-freq="233.08188075904496" name="midi_58" "="" style="--gcs:18" type="button>"></button>
<button aria-label="B3" data-freq="246.94165062806206" name="midi_59" "="" style="--gcs:19" type="button>"></button>
<button aria-label="C4" data-freq="261.6255653005986" name="midi_60" "="" style="--gcs:22" type="button>"></button>
<button aria-label="C#4" data-freq="277.1826309768721" name="midi_61" "="" style="--gcs:24" type="button>"></button>
<button aria-label="D4" data-freq="293.6647679174076" name="midi_62" "="" style="--gcs:25" type="button>"></button>
<button aria-label="D#4" data-freq="311.12698372208087" name="midi_63" "="" style="--gcs:27" type="button>"></button>
<button aria-label="E4" data-freq="329.6275569128699" name="midi_64" "="" style="--gcs:28" type="button>"></button>
<button aria-label="F4" data-freq="349.2282314330039" name="midi_65" "="" style="--gcs:31" type="button>"></button>
<button aria-label="F#4" data-freq="369.99442271163446" name="midi_66" "="" style="--gcs:33" type="button>"></button>
<button aria-label="G4" data-freq="391.99543598174927" name="midi_67" "="" style="--gcs:34" type="button>"></button>
<button aria-label="G#4" data-freq="415.3046975799451" name="midi_68" "="" style="--gcs:36" type="button>"></button>
<button aria-label="A4" data-freq="440" name="midi_69" "="" style="--gcs:37" type="button>"></button>
<button aria-label="A#4" data-freq="466.1637615180899" name="midi_70" "="" style="--gcs:39" type="button>"></button>
<button aria-label="B4" data-freq="493.8833012561241" name="midi_71" "="" style="--gcs:40" type="button>"></button>
<button aria-label="C5" data-freq="523.2511306011972" name="midi_72" "="" style="--gcs:43" type="button>"></button>
<button aria-label="C#5" data-freq="554.3652619537442" name="midi_73" "="" style="--gcs:45" type="button>"></button>
<button aria-label="D5" data-freq="587.3295358348151" name="midi_74" "="" style="--gcs:46" type="button>"></button>
<button aria-label="D#5" data-freq="622.2539674441618" name="midi_75" "="" style="--gcs:48" type="button>"></button>
<button aria-label="E5" data-freq="659.2551138257398" name="midi_76" "="" style="--gcs:49" type="button>"></button>
<button aria-label="F5" data-freq="698.4564628660078" name="midi_77" "="" style="--gcs:52" type="button>"></button>
<button aria-label="F#5" data-freq="739.9888454232689" name="midi_78" "="" style="--gcs:54" type="button>"></button>
<button aria-label="G5" data-freq="783.9908719634986" name="midi_79" "="" style="--gcs:55" type="button>"></button>
<button aria-label="G#5" data-freq="830.6093951598903" name="midi_80" "="" style="--gcs:57" type="button>"></button>
<button aria-label="A5" data-freq="880" name="midi_81" "="" style="--gcs:58" type="button>"></button>
<button aria-label="A#5" data-freq="932.3275230361799" name="midi_82" "="" style="--gcs:60" type="button>"></button>
<button aria-label="B5" data-freq="987.7666025122483" name="midi_83" "="" style="--gcs:61" type="button>"></button>
<button aria-label="C6" data-freq="1046.5022612023945" name="midi_84" "="" style="--gcs:64" type="button>"></button>
<button aria-label="C#6" data-freq="1108.7305239074883" name="midi_85" "="" style="--gcs:66" type="button>"></button>
<button aria-label="D6" data-freq="1174.6590716696303" name="midi_86" "="" style="--gcs:67" type="button>"></button>
<button aria-label="D#6" data-freq="1244.5079348883235" name="midi_87" "="" style="--gcs:69" type="button>"></button>
<button aria-label="E6" data-freq="1318.5102276514797" name="midi_88" "="" style="--gcs:70" type="button>"></button>
<button aria-label="F6" data-freq="1396.9129257320155" name="midi_89" "="" style="--gcs:73" type="button>"></button>
<button aria-label="F#6" data-freq="1479.9776908465378" name="midi_90" "="" style="--gcs:75" type="button>"></button>
<button aria-label="G6" data-freq="1567.981743926997" name="midi_91" "="" style="--gcs:76" type="button>"></button>
<button aria-label="G#6" data-freq="1661.2187903197805" name="midi_92" "="" style="--gcs:78" type="button>"></button>
<button aria-label="A6" data-freq="1760" name="midi_93" "="" style="--gcs:79" type="button>"></button>
<button aria-label="A#6" data-freq="1864.6550460723597" name="midi_94" "="" style="--gcs:81" type="button>"></button>
<button aria-label="B6" data-freq="1975.533205024496" name="midi_95" "="" style="--gcs:82" type="button>"></button>
<button aria-label="C7" data-freq="2093.004522404789" name="midi_96" "="" style="--gcs:85" type="button>"></button></div>
</form>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Orbitron:wght@400;900&display=swap');
.kb {
  block-size: var(--_bs, 8em);
  display: grid;
  grid-column-gap: 1px;
  grid-template-columns: repeat(var(--_r, 156), 1fr);
  grid-template-rows: repeat(5, 1fr);
}
.kb [aria-label] {
  --_focus-bxsh: inset 0 0 50px #fff, inset 20px 0 80px hsl(var(--_h,0), 70%, 35%), inset -20px 0 80px hsl(var(--_h,0), 100%, 50%);
  --_hover-bg: linear-gradient(to bottom, #FFF 0%, hsl(0, 0%, 90%) 100%);
  background-color: var(--_bgc, #FFF);
  border: 0;
  border-radius: 0 0 3px 3px;
  grid-column: var(--gcs) / span var(--_csp, 3);
  grid-row: 1 / span var(--_rsp, 5);
  position: relative;
}
.kb [aria-label*="#"] { 
  --_csp: 2;
  --_focus-bxsh: inset 0 0 50px hsl(var(--_h,0), 25%, 35%), inset 20px 0 80px hsl(var(--_h,0), 70%, 35%), inset -20px 0 80px hsl(var(--_h,0), 100%, 50%);
  --_hover-bg: linear-gradient(to top, #000 0%, hsl(0, 0%, 25%) 100%);
  --_rsp: 3;
  background-color: var(--_bgcs, #000);
  z-index: 1;
}
.kb [aria-label]:not(.keydown):hover { background: var(--_hover-bg); }
.kb [aria-label]:not(.keydown):focus-visible { box-shadow: var(--_focus-bxsh); outline: none; }
.synth {
  background: var(--synth-bgc, linear-gradient(215deg, hsl(0, 0%, 25%), hsl(0, 0%, 15%), hsl(0, 0%, 5%)));
  border-radius: .33em .33em .75em .75em;
  color: var(--synth-c, #FFF);
  font-family: var(--synth-ff, 'Russo One', sans-serif);
  inline-size: max-content;
  padding: .5em 1em 1em 1em;
}
.synth h2 {
  font-size: var(--synth-h-fz, 1.33em);
  font-weight: var(--synth-h-fw, 700);
  margin: 0 0 .5em 0;
}

/* modifiers */
.kb--61 { --_r: 108; }
.kb--49 { --_r: 87; }
.kb--32 { --_bs: 6em; --_r: 57;}
.kb--25 { --_bs: 6em; --_r: 45; }

.keydown {
  --_l: calc((var(--v, 1) * 0.394%));
  --_bgc: hsla(var(--_h), 50%, calc(95% - var(--_l)));
  --_bgcs: var(--_bgc);
}
.keydown::after {
  background-color: #000;
  block-size: .5em;
  border-radius: 50%;;
  content: "";
  display: block;
  inline-size: .5em;
  inset-block-end: 1em;
  inset-inline-start: 50%;
  position: absolute;
  transform: translateX(-50%);
}
.chord {
  --synth-bgc: hsl(351, 68%, 30%);
  --synth-ff: 'Orbitron', sans-serif;
  --synth-h-fz: 1em;
  --synth-h-fw: 900;
  --_h: 351;
  --v: 70;
}
.chord fieldset {
  border: 0;
  display: flex;
  gap: 1em;
  padding: 0 0 1em 0;
}
.chord label {
  align-items: center;
  border: 1px solid hsl(351, 68%, 90%);
  border-radius: .2em;
  display: flex;
  gap: .5em;
  padding: 1ch;
}
.chord select {
  /* -webkit-appearance: none; */
  background: none;
  border: none;
  color: inherit;
  font-family: inherit;
  font-size: inherit;
  outline: none;
}
.chord label:has(select:focus-visible) {
  background: hsl(351, 68%, 90%);
  color: hsl(351, 68%, 10%);
}
!

JS 

              
                /* keyboard.js */
const getHz = (N = 0) => 440 * Math.pow(2, N / 12);
const notes = ['A', 'A#', 'B', 'C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#'];

function getNote(N) {
  const key = N % 12;
  const note = notes[key < 0 ? 12 + key : key];
  const octave = Math.ceil(4 + (N / 12)); /* 4 is octave of root, 440 */
  return {
    freq: getHz(N),
    midi: N + 69, /* A4 === MIDI note number 69 */
    note,
    octave: note === 'B' || note === 'A#' ? octave - 1 : octave
  }
} 

function keyboard(start = -48, end = 40, chord) {
  const data = freqs(start, end, chord);
  return data.map(item => `<button aria-label="${item.note}${item.octave}" data-freq="${item.freq}" name="midi_${item.midi}"" style="--gcs:${item.offset}"${item.highlight ? ` class="keydown"`:''} type="button>"></button>`).join('\n') 
}

/* chord.js */
const chords = {
  'Major triad': [0, 4, 7],
  'Minor triad': [0, 3, 7],
  'Augmented triad': [0, 4, 8],
  'Diminished  triad': [0, 3, 6],
  'Dominanth seventh': [0, 4, 7, 10],
  'Major seventh': [0, 4, 7, 11],
  'Minor-major seventh': [0, 3, 7, 11],
  'Minor seventh': [0, 3, 7, 10],
  'Augmented-major seventh': [0, 4, 8, 11],
  'Augmented seventh': [0, 4, 8, 10],
  'Half-diminished seventh': [0, 3, 8, 10],
  'Diminished seventh': [0, 3, 6, 10],
  'Dominant seventh flat five': [0, 4, 6, 10],
  'Major ninth': [0, 4, 7, 11, 14],
  'Dominant ninth': [0, 4, 7, 10, 14],
  'Dominant minor ninth': [0, 4, 7, 10, 13],
  'Minor-major ninth': [0, 3, 7, 11, 14],
  'Minor ninth': [0, 3, 7, 10, 14],
  'Augmented major ninth': [0, 4, 8, 11, 14],
  'Augmented dominant ninth': [0, 4, 8, 10, 14],
  'Half-diminished ninth': [0, 3, 6, 10, 14],
  'Half-diminished minor ninth': [0, 3, 6, 10, 13],
  'Diminished ninth': [0, 3, 6, 10, 14],
  'Diminished minor ninth': [0, 3, 6, 10, 13],
}

function getChord(key, chord) {
  const index = notes.indexOf(key);
  return chords[chord].map(note => getNote(note + index))
}

/* audio.js */
const audioCtx = new (window.AudioContext || window.webkitAudioContext)
const gainNode = audioCtx.createGain()
const notemap = new Map();
gainNode.connect(audioCtx.destination);

function createOscillator(freq) {
  const oscNode = audioCtx.createOscillator()
  oscNode.type = 'triangle'
  oscNode.frequency.value = freq
  oscNode.connect(gainNode)
  return oscNode
}

function noteon(key) {
  if (!key.classList.contains('keydown')) {
    gainNode.gain.value = 0.33
    notemap.set(key.name, createOscillator(key.dataset.freq))
    notemap.get(key.name).start(0)
    key.classList.add('keydown')
  }
}

function noteoff(key) {
  key.classList.remove('keydown')
  const oscNode = notemap.get(key.name)
  if (oscNode) oscNode.stop(0)
  notemap.delete(key.name)
}

function playChord(app, notes) {
  app.querySelectorAll('.keydown').forEach(key => key.classList.remove('keydown'))
  gainNode.gain.value = 1 / notes.length
  notes.forEach(note => {
    const key = app.elements[`midi_${note.midi}`]
    key.classList.add('keydown')
    const oscNode = createOscillator(note.freq)
    oscNode.start(0)
    oscNode.stop(audioCtx.currentTime + 1.5)
    gainNode.gain.setTargetAtTime(0, 0.75, 0.25)
  })
}

/* init */
app.addEventListener('submit', event => event.preventDefault());
app.addEventListener('change', () => {
  const notes = getChord(key.value, chord.value);
  playChord(app, notes)
})
/* safari hack */
app.addEventListener('pointerdown', () => audioCtx.resume());
!
999px

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