What the hell am I even talking about? Back to Loudness Metering...

When television shows are mixed, it is common practice to have a loudness meter (metering the mix bus) visible at all times. The mixer constantly watches to make sure the mix is sitting around -23 LUFS (loudness units relative to full scale). Of course, with television, the vast majority of your LUFS allowance is going to be blown on dialog. So really, the meter is mostly telling the mixer if the dialog is at a consistent level (for example, whispered dialog shouldn't actually be quieter than spoken dialog as the timbre conveys the concept to the audience). If a scene has an extended music passage without dialog, the level that music often sits at is – you guessed it – about -23 LUFS.
Mixing music is different, of course; but for a conventional song, the focal element at any given point in the song is at the same level. The most obvious example is an instrumental solo that replaces the lead vocal for a verse. If you've got your mix with the vocals hovering at -14 LUFS, and most elements remain the same, you can turn up the solo instrument until you get back to -14 and you'll probably have a pretty good mix. An instrumental intro usually works out in a similar manner. This is why I started to mix music with a loudness meter across the master bus. My monitor volume is already set (see "Monitor at a Consistent Volume When Mixing" in Tape Op #121), so I can be pretty sure – just by turning things up until they sound right – that my mix will be sitting close to the magic -14 LUFS without even looking. But the loudness meter helps me ensure that different parts of the same song are somewhere close to one another.

But what if you don't want the volumes to be close to each other? Even if you're mixing a song that is purposefully loud/quiet/loud, the loud parts all probably want to be about the same loudness, and the same for the quiet parts. Looking quickly at Spiritualized's "All of My Thoughts," the three quiet sections all average around -14 LUFS, and both loud sections averaging around -7 LUFS. The whole song averages about -11 LUFS, which means any streaming service will probably offset the playback by about -3 dB.
Whether you like it or not, the overall average for your entire song will end up at -14 LUFS. Having a loudness meter helps you 'hear' this reality. So, mix with dynamics, but make your mix even better by being aware of the fact that if you have a massive, loud, 64-bar, full-band outro, the front part of the song, which is just voice and piano, will get turned down to inaudibility if you don't build your mix to account for it. See you in the sweet spot!
Thanks to Patrick Klem for reading a draft of this article and offering his insights and edits.