You can’t talk about housing these days without talking about affordability and how elusive it has become for so many people — particularly those at the lower end of the income spectrum. It’s no surprise then that affordability is rapidly becoming a topic on the presidential campaign trail. But what defines affordable? There is a definition, or at least a guideline: A long-held rule of thumb is that housing costs shouldn’t exceed 30% of your income. So who came up with that?

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