Basic Guidelines For English Spellings
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1(forming adjectives) able to be.
2(forming adjectives) to be.
3(forming adjectives) subject to.
4(forming adjectives) relevant to or in accordance with.
5(forming adjectives) having the quality to.
From French -able or Latin -abilis; originally found in words only from these forms but later used to form adjectives directly from English verbs ending in -ate, e.g. educable from educate; subsequently used to form adjectives from verbs of all types (influenced by the unrelated word able), e.g. bearable, saleable.