Let's run an experiment: at home, look up any concerts you have on video, of any kind, era or style... as long as they're "live." Look at the singer's microphone, and at the ones picking up the sound of the guitar amps. If you can, do the same, closely examining the mics set up all around the drum kit. Don't they often match? Curiously, that's the case whether the show is a band of friends at some party in some city in our country, or the latest stop on the tour of the number-one singer on the charts of who knows where.
For many, many years two microphones have taken the crown: the Shure SM58 and the Shure SM57. The first was, and remains, the undisputed king for vocals, while the tighter cardioid pattern of the SM57 is still the most sought-after for amps (especially electric guitar), snares and percussion. These two, together with the, at last, forgotten AKG C1000 (its reign has faded in favor of new and better additions), would let us handle any event, however demanding. I mean that with a handful of them we could mic every instrument. That's without counting DI boxes, those little transformers that have become indispensable on stage. And why is that?
Unlike the work done in a recording studio, the concept of tolerance margin in a live show is radically different. The unavoidable acoustic problems, and the need not only to cover large spaces but to reproduce those same sounds very close to the sources themselves (the musicians' monitors), complicate and, in a way, make the working techniques completely different. The ideal live microphone is the one that lets us pick up the sound of the original source with maximum fidelity and nothing else. And this simple premise is really damn hard (pardon the expression) to fulfill. Go back up on stage and pay attention, for example, to the mix of sounds a drum kit gives off, and this time take into account the drum fill, that big monitor the musician needs to be able to follow the band's rhythm accurately.
The SM58 and SM57 meet this premise very well (and have for decades). Especially the 58, capable of capturing the sound of any voice with perfect accuracy while also avoiding, for example, feedback from the monitors (both the close ones and the side fills). The success of the SM58 is that it's "perfect" for any voice, although we could find far more precise models for each individual singer.
Choosing the Right Mic
The professional market in our country has undergone a major change over the last decade, especially after 1992. The socioeconomic crisis Spain went through took its toll on an industry that needed a very deep cleanup, which has led to the disappearance of quite a few "fly-by-night" companies in favor of more serious and professional ones. Over the years, especially recently, when a certain interest in cultural events has been noticeable (there are fewer big concerts, but far more events), the ever-growing level of demand from audiences (they are starting to be critical of the sound) is allowing not only better and better equipment, but also broader possibilities even in fields as important yet overlooked until now as microphone selection.
The technical rider becomes indispensable when approaching a concert. The sound engineer asks for a list of microphones that he believes are ideal for miking all of the band's instruments and vocals. In recent years we've seen new microphones push aside others that had been untouchable until then, with models such as the AKG 451 or 3000, the Sennheiser 601, the ATM 35 and many more that are also more popular with sound reinforcement companies.
The microphone is the first electromechanical element in the sound chain. A bad choice can undoubtedly have a disastrous effect on every later step. It's no good picking a mediocre model (much less the wrong one) and then compensating for its supposed shortcomings with EQ and dynamics processing (sometimes even with stratospheric digital processing, that is, reverbs and delays everywhere). It's worth paying attention, then, to choosing them, taking several factors into account, the main ones being the type of instrument, its sound and the sound we want to achieve. Mastering all three elements is really a matter of time and learning and, most of the time, comes through the effective but slow process of trial and error.
I'm now going to stand up for sound reinforcement companies. Although the ideal is always to get what we ask for in the technical rider, the company in charge of the PA often doesn't have the requested microphone and therefore offers us an alternative (what's known as a counter-rider). Some companies offer an alternative even when they do have the requested microphone, especially when they know the venue and their gear. It's worth listening to them and, at the very least, weighing their offer: most of the time they tend to be right.
Proper Placement
At a more or less serious concert we already find the figure of the mic tech. As the name says, he is in charge of placing and managing each and every one of the microphones we'll use during the show. He must be a professional, because if the microphone is the first element in our production chain, its placement is, without a doubt, the second important decision. 
No matter how good the microphone is, how good the instrument is and how perfect the choice (let's call it synergy), placing it even a couple of centimeters off will surely be another source of disasters. But anyone who thinks the mic tech is to blame for a badly placed microphone is surely mistaken. Again, it is the band's engineer who is solely responsible for that, and who must take his time before the soundcheck to verify that the microphone is exactly where he wants it. For example, it is well known that the AKG D112 is one of the most requested for kick drums. It picks up the low-end sounds of the shell very well, while doing the same with small details in the highs, without the dynamics being compromised even by very strong transients. But its placement on the instrument itself will always give us a different sound, even with differences of barely a few centimeters or a few degrees of angle. The mic tech doesn't know (and has no way of knowing) what sound we want: more or less head, more or less attack, more or less low end, and so on.
Discordant Elements
Experience is undoubtedly a hugely important learning element. The years and the gigs help us improve a little more every day, and we rarely do a job without learning something new. Sometimes the usual formulas are compromised by some discordant element and we need to take an action we would never think of elsewhere. That's normal.
Acoustics remains a topic of conversation and study even when choosing microphones. Strange as it may seem, a low ceiling or the bounce of PA sound off a relatively close back wall can lead us to choose one microphone over another. Obviously, many problems of this kind can be solved at the console, though for that we need the cooperation of the microphone itself. For example, the versatile, all-purpose SM58 allows us, if the singer has a good level of vocal delivery, to reduce its gain to avoid ambient sound getting in as "direct" sound. Of course, we'll then have to raise the volume level higher, for both the PA and the monitors.
Monitors also tend to be problematic in their pairing with microphones. A singer will want to hear himself perfectly in his monitor, which leads us to choose microphones that reject all sounds arriving from the rear (the SM58 is an ace in this field). Today it's relatively easy to mix a singer's monitor if we have a good EQ and a suitable monitor wedge. But sometimes we're asked for some exaggerations involving microphones that aren't inclined to be the stars of a monitor mix: ambient mics. It's very common, for example, for percussionists to ask for even the smallest details of a cajón or an ambient kit to be reproduced equally in their own monitor. Here it is hard for the ambient mic to avoid annoying feedback, and for the PA engineer it will surely cause bigger headaches: not only does he have the "direct" sound, but a slight copy of it coming from the monitor. Back to square one: selection and better placement. There are tricks to solve this, such as using two different microphones (one for the PA, another for monitors) or better aiming the ambient microphones and the monitor itself. Experience.
Conclusion
Let this article serve as a short orientation guide, especially for beginners. We intend to review, little by little, the different standard microphones for each of the most common instruments at a live event, as well as their placement and other essential tricks. But I'd like to make it clear that the microphone is one of the main essential elements for making our mix as good as it can be. You know the rule about the quality of a system: in general, its maximum quality will be exactly that of its lowest-quality component. And although it may seem "strange" in a world as consumerist as ours, sometimes the most expensive microphone isn't the best, but the most suitable one.
