I suppose I disagree with that style guide itself, then. Thinking about which quotes to use should be rare. The default choice should be double quotes. One should use single quotes only in the rare instance in which disabling interpolation is desired. It's far more often that interpolation is desired -- or that the developer doesn't care. In a language where there's a performance cost to using double quotes, the proper stylistic preference would be toward careful management of quoting. But it just doesn't make sense to impose such a burden in Ruby. A style guide should have solid reasons supporting each rule. Arbitrary rules erode its utility.