Updated: Jan 18
Most people have heard of the term "pan law", but many people don't fully understand what it is or why it matters to them. So we're here to define pan law in detail. Pan law determines what happens when a mono channel is routed to a stereo bus (either a group or the "master" 2-bus). The designator (0dB, -3dB, -6dB, etc) describes how much the audio is attenuated (made quieter) when it's panned to the centre. More specifically, it determines what happens when mono channels are not panned hard-left or hard-right (that is, they're panned somewhere in the middle). In (almost) all cases, a mono channel panned hard-left or hard-right will come through to (one side of) the stereo bus at exactly the same level as it is on the channel. 0dB Pan Law 0dB pan law is probably the easiest to understand. With this pan law, mono channels that are panned centre are not attenuated at all. Audio that's at full scale digital (0dBFS - very loud!) would appear at the stereo bus at the same level - full scale 0dBFS - on both the left and the right side of the bus. Another way to think about this is to imagine panning a mono channel from hard-left to hard-right. When it's panned hard-left, the left side of the stereo bus is showing the same level as the mono channel. As you pan towards centre, the right side of the stereo bus starts to raise in level, but the left side doesn't lower. At centre, both sides of the stereo bus are at the same level as the mono channel. As you then pan towards the right, the left side of the stereo bus lowers in level. But the right side stays at the same level. The advantage to using this pan law is that mono channels are as loud as stereo channels (stereo channels are just two mono channels panned hard-left and hard-right). This is because the mono channel "fills up" the stereo bus as if it's a stereo channel. This could be useful if you regularly work with a mix of stereo and mono channels and don't do much panning. It might also be worthwhile if you regularly use mono channels for foreground instruments (such as lead vocals or kick drum) because with 0dB pan law, mono channels are actually louder when panned centre than when they're panned to the side. This is because mono channels panned centre are sent to both speakers at full level. The disadvantage with 0dB pan law is that mono channels are much louder when panned centre than when panned to the side. This can make pan automation sound strange. It might also be a problem if you mostly mix with instruments panned center (as I do), leaving the panning to the later stages of the mix. With 0dB pan law, audio will actually get quieter if you pan it away from centre, which can unbalance the mix because those instruments are pushed further to the background. Of course, you'd compensate for this by adjusting the fader at the same time, but it's more work. Another disadvantage to 0dB pan law is that if you control the width of a stereo channel by adjusting the pans of the left and right sides, you'll find that the audio gets louder as you collapse it to mono. This is because each side gets added to the other side, but the level is not automatically reduced to compensate. In most cases this sounds "wrong". 0dB pan law is probably useful for modern electronic or pop music, where powerful foreground synths are in stereo but highly correlated (mostly mono anyway), and there's a strong emphasis on the foreground sounds that are panned centre. -6dB Pan Law By contrast, -6dB pan law attenuates the mono channel by 6dB when it's panned mono. If you pan a mono channel from hard-left to hard-right, it'll start at full level at hard-left. As soon as you start moving it to the right, however, the left side of the stereo bus starts lowering in level at the same time that the right side of the stereo bus is increasing in level. At centre, the audio is coming through both sides of the stereo bus, but at half the level. As you then move the audio to the right, the left side of the stereo bus continues lowering in level and the right side continues rising in level. When the channel is finally panned hard-right, the signal coming through the right side of the stereo bus is at the same level as it is on the channel. The advantage to using this pan law is that mono channels stay the same level wherever they're panned. This should make pan automation sound more natural, and make it easier to pan mono channels late in the mix without changing the balance much. The other advantage is that stereo channels in which the main component of the sound is equal on both sides (i.e. mono) can have their width adjusted by panning each side without the apparent level changing. The disadvantage is that in a full mix it can make a mono channel actually sound weaker in the centre and louder at the sides. This is because most mixes are more dense in the centre than at the sides. So even at the same level, a mono channel can get a bit lost, or "masked" in the center, but come through clearly at the side. This is the exact opposite behavior to 0dB pan law, where mono channels in a mix are louder in the centre but can get lost at the sides. Also worth considering is that with -6dB pan law, stereo channels can actually sound up to 6dB louder than mono channels at the same fader setting. Another disadvantage relates to stereo channels that are loosely correlated (true stereo). These are stereo channels where each side is similar, but slightly different (for example, ambience effects or wide stereo synths). If you try to adjust their width by panning each side using -6dB pan law, they may actually get quieter. This is because each side is not exactly the same, and thus they won't exactly reinforce each other to make up for the 6dB gain reduction. This is also what happens when you use a separate plugin to collapse the stereo field to mono. Most such plugins use a -6dB pan law and thus the sound itself gets smaller. -6dB pan law is probably useful for more ambient or acoustic music where there's more emphasis on background textures (and stereo space), and foreground sounds don't dominate the mix. -3dB Pan Law -3dB pan law is a compromise between 0dB and -6dB pan laws. Mono channels are attenuated when panned centre, but not as much as with -6dB pan law. When listened to in solo, there's still a slight increase in level as the channel is panned towards the centre. When panning in a mix, however, the level remains mostly even and the mix retains its balance better. Fully correlated stereo channels (where both sides are exactly the same - effectively mono) will become slightly louder when collapsed to mono using pan controls for each side. But loosely correlated stereo channels (where each side is slightly different - true stereo) will remain about the same level no matter how each side is panned. -3dB is probably your best choice if either 0dB or -6dB pan laws don't feel right to you, or you find yourself regularly working in a variety of different styles and genres. Why It Doesn't Matter... Much Mixers are complex, but so are mixes. When you're actually working on a mix, you're adjusting faders as well as pans anyway. Using a pan law that works for you merely reduces some of the work in adjusting the volume as you place a sound in the stereo space. Additionally, each mix is different. The way audio signals combine to reinforce and mask each other is subtle and complex. Regardless of which pan law you use, you have to use your ears to determine if you need to adjust the fader to compensate for any actual or perceived volume change. Other Weird Pan Laws 0dB, -3dB, and -6dB are the basics. But some other digital mixers have other options. -2.5dB, -4.5dB, -3dB compensated, constant power, log/sin taper, and others are some examples of pan laws on offer in some mixers and DAWs. You'll have to check the manual or other documentation to find out how these differ from the usual pan laws. Armed with this article, you should be able to make sense of it all and choose the best option for you!  I'm not a Logic user, but apparently the "3dB compensated" pan law in Logic boosts hard-panned mono channels by 3dB. Look out!  Depending on correlation. A stereo channel will sound 6dB louder than a mono channel with -6dB pan law if both sides of the stereo channel are exactly the same (they're fully correlated). If each side is slightly different (that is, the stereo channel is loosely correlated), then it will sound slightly less than 6dB louder than the mono channel.