So there are a few exceptions to this rule, but overwhelmingly rivers converge as they flow to the sea. Tributaries will join with the main river flow and that main flow, always moving in the easiest path downhill, will never split until it reaches the ocean.

Countries near to one another and peacefully coexisting tend to adopt the same or similar systems of government. Unless the area is currently undergoing a period of upheaval, having a functioning democracy next to an autocracy is unlikely unless these two countries are antagonistic in their relationship. When worldbuilding don’t worry about have kingdom after kingdom after kingdom. In addition, countries with similar government types tend to be friendlier with one another, while those with opposing styles of government tend to have more strained relations.

Borders between countries will often fall along easily identifiable and defendable natural boundaries, such as rivers, mountain ranges, oceans, deserts, etc. Straight borders may come about because of political action and agreement between parties. Borders that are randomly drawn with no consideration for the natural terrain or the political history of area should generally be avoided unless your world contains a lot of drunk cartographers.

It takes a lot of food to feed a population, and everyone dies without water. Oftentimes, however, worldbuilders will forget to include the means of food production or water sources in their worlds. Sometimes it’s not important to a story, other times it is. But try to consider how that city gets its meals, how those intrepid planetary colonists sustain themselves, or how that army feeds itself.

Often bathrooms can be waved away as places that exist as an assumed element in fictional worlds. In worlds with actual set pieces or well defined locations, however, the absence of restrooms does raise questions. This is particularly important in small spaces with well-defined layouts, like an apartment, a castle, or the crew quarters of a spaceship.

Jungles next to deserts next to glaciers near lavalands. While this may be acceptable in whimsical or magical worlds, many worldbuilders opt for a slightly more realistic take on how their biomes and terrain are located. Is mapping sea currents and a well-studied take on tectonic plates necessary? Well that’s up to you.

Planets can be complex places. Making each planet one dimensional (a desert planet, a jungle planet, a tropical water planet) is great for a space opera but may be less great for a realistic hard sci-fi setting. Though there are exceptions to this, for example gas giants like Jupiter, ice moons like Ganymede and Enceladus, and rocky desert worlds like Mars are all real locations that have what some might describe as a single dominant biome.

Cities are most often located near natural resources, fertile farmlands, strategically important corridors, culturally significant locations, or trade routes. Instead of placing cities on a map because there’s empty space, consider why people would have wanted to settle there in the first place.

When map making, many worldbuilders will try to fill up any and all empty space with something. It’s okay to have large swathes of empty grasslands. You don’t need to put a city in the middle of it. It’s okay to have a large expanse of empty space. You don’t need to fill it with habitable planets. Many real-world features are not homogeneously distributed and yours do not need to be either. Empty spaces are okay.

How long does it take to walk 100 miles? 10 minutes maybe? How far is forty feet? Like the length of a car? Worldbuilders often struggle with scale, and this is because of two reasons: not being familiar with the scale of real world elements, and having an issue assigning real world values to fictional things. Afterall, the imagination is not easily measured or quantified. How do you accurately determine the distance between different fictional towns or the length of fictional eras? Creating maps and timelines will help you with this. But once you have decided these towns are 100 miles apart or that an era lasts 1000 years, how do you figure out how long it takes to walk between them or how many generations have passed in that timeframe? This latter knowledge is something that needs to be acquired and honed.

Particularly in narrative worlds, creators need to ensure that prior events have the expected impact, and that the impact is proportionate. A character experiencing extreme trauma won’t recover in a day. A person with a grievous wound won’t heal in 20 minutes. And a country that has been fragmented by civil conflict won’t forget that past in a single decade.

Elements can be tied to real world things and thus do not make sense to exist in a fictional world without those equivalents. In a world without Jesus, people would not say ‘geez.’ In a world without France, bubbly alcoholic beverages would not be called champaign. There are exceptions to how immersion breaking this can be, however. Most people do not know the origin of the term mausoleum, and so having mausoleums in a world where Mausolus, ruler of Caria, never existed raises less eyebrows.

Not only do different cultures differ greatly, there is also tremendous variability in a single culture. There may be subgroups, counter-cultures, new and old trends mixed together, and all other manner of variation. Having an entire culture of people who are ‘proud warriors’ or ‘cold-minded scientists’ can be a useful archetype to begin with, but not extrapolating from this creates a one-dimensional, monolithic culture. This may be fine for some types of worlds but will not be for others. If you are trying to create nuanced, ‘realistic’ cultures, avoid painting entire communities with a single brush.

If you have a creature that explodes at the slightest touch, people will have long ago figured out how to use them for warfare. If you have crystals that float, people will use those as a means of transportation. And if you have both creatures that explode at the slightest touch and magically floating crystals, aerial mines will certainly exist. With each new element you incorporate into a world, particularly with fantastic and magical ones, consider how the greater world is influenced. Many worldbuilders fail to do this, in part because it’s difficult to imagine how everything can and will interact. The only remedy for this is to spend as much time as possible thinking about how each part of your world fits together.

Associated elements in worldbuilding are things that should be present, or are likely to be present, given the inclusion of another element. For example, if there is oceanic trade in a fictional world there are likely to be charts and maps that exist in that world as well. Some associated elements are requisite. Having one requires having the other unless some other magical or fantastic force is also present. Like wine requires grapes or parachutes require air resistance. Ignoring or not understanding requisite associated elements can lead to trouble. It is how you wind up with subterranean jungles full of plants. Plants that require light. Of which there is little underground. Or bronze-age civilizations of aquatic mermen. Bronze requiring a forge to be cast. Forges that require fire. Of which there is little underwater. While certainly in some cases these issues may not impact immersion, as a worldbuilder it is always best to be aware of them and then decide for yourself whether it is worth your time or not to craft a solution.