there is a fine line between

there is a fine line between (something and something else)

There is little difference between something and something else. There is a fine line between a frown and a grimace.
See also: fine, line, there
References in classic literature ?
On this twenty-fifth of March, it was John Willet's pride annually to settle, in hard cash, his account with a certain vintner and distiller in the city of London; to give into whose hands a canvas bag containing its exact amount, and not a penny more or less, was the end and object of a journey for Joe, so surely as the year and day came round.
But the air of smartness, the cock of the hat to which John Willet had objected, and the spring nosegay, all betokened some little errand of his own, having a more interesting object than a vintner or even a locksmith.
Andrew Lorraine knows there is a fine line between Triple-A and the big-leagues.
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