(NEXSTAR) – Once you see it, you can't unsee it. Examples of hostile architecture, also known as hostile design or defensible architecture, are everywhere in cities and public spaces.

You've likely seen it outside businesses, at bus stops, along sidewalks, and in parks, as it can take many forms. Hostile architecture is "hostile" because it subtly (or sometimes quite plainly) keeps people away.

"Through design you are making them uncomfortable so that they leave, or you're making particular activities that they tend to do uncomfortable," explained Anastasia Loukaitou-Sideris, a professor of urban planning and dean at UCLA's Luskin School of Public Affairs.

One common example of hostile architecture is simple: a bench divided in two, usually to discourage someone from lying down. Even the makeup of the bench (slats of metal with gaps in between) is comfortable enough for a short sit, but is a subtle way to discourage lingering for hours.

Some transit stations eliminate benches altogether, offering riders a surface to lean against instead.

Other pieces of hostile design are more obviously hostile in appearance. Metal spikes, for example, can be seen along sidewalks or under awnings to prevent people from sitting or lying on the ground.

But sometimes it's less obvious, like when large, steel planter bins were installed on San Francisco sidewalks. Residents of the neighborhood said it wasn't just for beautification, but also to leave less room for people to set up tents on the sidewalk.

Another San Francisco neighborhood tried to do the same thing

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