"Ruthless" - is there an name for words like this?

There are some words which look as though they are constructed as opposites to a root word, but we never (or hardly ever) use the root by itself. We describe people as 'Ruthless' but don't use 'Ruthful'. Another example is 'disjointed' - you might describe what somebody said as... show more There are some words which look as though they are constructed as opposites to a root word, but we never (or hardly ever) use the root by itself. We describe people as 'Ruthless' but don't use 'Ruthful'.

Another example is 'disjointed' - you might describe what somebody said as 'disjointed' but has anybody ever called a sentence 'jointed'?

I've seen a website once with a list of this type of word, but I've lost the URL - anybody know it?

Surely, with so many odd words in English to describe aspects of the language itself, there is a posh word for these 'orphan opposites'.
Update: Sorry about the typo in the heading/question - whoops.
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