296: -'d Contraction for Should, not Could Sep 30, 2015

'Should', 'would' and 'could are all modal verbs, all ending in 'ould'. Still, the common contraction of -'d should only used for 'should' and 'would' (i.e. he'd, they'd) but not for 'could'. For something as colloquial as contractions, there is no rule, except for commonality.