osu! is primarily a music game, and beatmaps are usually created with the idea that they fit the music, rather than fitting a certain BPM. In this short essay I will be discussing music theory as it applies to an osu! beatmap, as well as explaining why certain things sound bad (at least to me). In no way can the matters discussed even be considered “guidelines”. Instead, think of it as theory. Most of the time they will apply to some parts of songs, but there are many exceptions. Everything I say here is personal opinion based upon 12+ years of concert band experience or taken directly from Wikipedia. It can’t be applied to all forms of music, especially avant garde stuff and most Touhou music (so, half of osu?)
The downbeat is the impulse that occurs at the beginning of a bar in measured music. Its name is derived from the downward stroke of the director or conductor's baton on the first beat of each measure. It frequently carries the strongest accent of the rhythmic cycle.
A back beat, or backbeat, is a syncopated accentuation on the "off" beat. In a simple 4/4 rhythm these are beats 2 and 4… In today's popular music the snare drum is typically used to play the backbeat pattern. This is why putting claps on beats 2 and 4 sounds good. Most contemporary music uses a snare backbeat. Some songs will add a kick to the backbeat by moving the second beat up by half a beat (so beats 2 and 3.5).
This is where the downbeat (usually) is omitted, or dropped. By dropping the beat in a faster song, tension is usually created. Often times only one particular instrument will drop the beat, when others play as normal. Dropping the beat can be an interesting way to make up a rhythm, but should not be overused. Another way to simulate this is by using upbeat or offbeat sliders.
On Beat Sliders
These are the most common sliders you will find. They play well, are easy to predict, and occasionally bland. Note how the slider starts on beats 1 and 3, which are the downbeats.
This is what I like to call them. Sliders which start on beat 4 have a serious problem associated with them: if it’s a 1/1 slider, it will end on the downbeat. That leaves the downbeat unstressed, and can be awkward to play, especially when repeated.
I call sliders which start on red ticks offbeat. These are especially dangerous because they usually leave you without a steady beat. Try to avoid repeating them, as they also suffer from the same complications as upbeat sliders.
2x+ Repeating sliders
Repeating sliders can be very interesting, but often times people will add multiple repeats on them. I consider repeating sliders which have more than 1 repeat to be confusing, because often times the 4th repeat does not show itself until you have already hit the slider itself. Short sliders and long sliders do not have this problem because short sliders can usually be predicted easily, and long sliders give you enough time to react. There are very few instances where a 2x repeating slider works better than 2x regular sliders or 4x circles.
One obvious exception is in long streams where a repeating slider is used to replace 4 circles. This is probably better than using 1x repeating sliders.
Alternating circle, slider, circle, slider is a neat way to map dotted half note rhythms (ie, 1 and 1/2 rhythms) because it puts the stress on the slider, which is usually the stressed note. I personally love these rhythms, and prefer them over 1x repeating sliders. You can also do circle circle slider circle slider slider, etc… It’s as simple as mixing up a straight 1/1 or 1/2 rhythm by stressing certain notes using sliders in different places.
Short Sliders vs Long sliders
Sliders in osu! most closely resemble a held note in music since spinners are rarely used and circles have no length. In this example you can see how the short sliders put the note that the player has to hit on a 1/4th note. Not only is this unintuitive since there is nothing to hit on the beat, but if you use long sliders instead, it will sound the same, hit on the beat, and will probably follow the music better. In general, short sliders are a bad idea. The reverse is true of very long sliders, but usually only because they end up passing over important parts of the song or are just boring. There are many exceptions, especially if the rhythm repeats and some variation is needed in the beatmap.
Music Theory Important
Most music works in groups of 2 or 4. 4 beats to a measure, 4 measures to a phrase, etc… So long as you place a slider beginning or a circle on the downbeat (long white tick) and sometimes the middle of the phrase, you can put in any number of upbeat or offbeat sliders with crazy slider patterns, stupid short sliders and silly streams in the map, even if it doesn’t fit the song. I’m serious. It’s obviously not entirely recommended, since then you can take any song with the same BPM and song structure, copy/paste it, and have the same low quality map. It’s also important to map to the music, but most music just repeats itself over and over again, so it’s good to throw in something different from time to time.
It’s common knowledge that when playing the bass line with any instrument, you can make up almost any rhythm and play any note so long as you play the downbeat in every measure in key. That’s how important the downbeat is. Obviously it’s not always perfect, but it’s at least acceptable.
Songs which only use onbeat sliders are destined to be boring, so make sure you improvise your rhythms.
My definition of overmapping is placing a note or slider end where no note exists in the background. There are a few reasons to overmap:
Chances are you’re mapping it wrong then.
If you really want to overmap something due to difficulty/fun reasons (the most common excuse) then make sure that there is not some other way to map the notes correctly (like using a repeating slider). Mapping a 1/6 snap roll with a 1/4 or 1/8 repeating slider is just plain wrong. They all play identically, and the only difference is how it sounds and the impact on the score.