When describing the Euclidean algorithm in his book, Elements, Euclid says the following:
When the less of the numbers $a$ and $b$ is continually subtracted from the greater, some number is left which measures the one before it.
So when looking for a GCD of, say, $25$ and $15$, we can subtract $15$ from $25$ and get $10$. So $10$ is the number left which should measure(divide) the one before it. It measures neither $15$ nor $25$.
Can anyone interpret what Euclid wanted to say with that sentence?