What does BPM mean?
Beats per minute – how many beats are played in one minute. At 60 BPM a beat falls every second; at 120 BPM every half second. Scores usually write it as a note value, such as ♩ = 120, and this metronome treats one BPM click as one quarter note. In compound time such as 6/8, where the printed tempo often counts eighth notes, entering that number directly will feel far too fast.
Will the tempo drift over time?
No. The browser's setTimeout and setInterval can be tens of milliseconds late, so triggering sound directly from them makes the beat wobble. This metronome uses look-ahead scheduling: every 25 ms it books any click due in the next 100 ms at an exact Web Audio clock time, and the audio hardware plays it. The spacing between beats stays constant even while the tab is busy doing other work.
What are subdivisions for?
They split each beat into smaller, quieter clicks: two for eighth notes, three for triplets, four for sixteenths, with the beat itself still accented. They help you keep your place between slow beats and internalise off-beat or shuffle feels. Once a passage is comfortable, switch back to quarter notes and see whether you can keep counting the smaller units in your head.
How does tap tempo work?
Tap the button on the beat, in time with the music or the rhythm in your head. From the second tap it starts averaging the recent intervals to work out a BPM, and four or more taps make it settle. Leaving a gap of more than two seconds resets the measurement, so you can simply start tapping again. The resulting tempo lands in the number box, where the ±1 buttons let you fine-tune it.