Scabby Doug asked why Scots has two distinct vowel qualities for 'pried' vs 'pride' and 'sighed' vs 'side' etc.... it's because the vowel crosses a morpheme boundary (oo, big words!), that is, 'sighed' is made up of two bits of meaning - 'sigh' itself plus the past tense indicator '-ed', whereas 'side' is a single morpheme. So in Scots or Scottish English you can tell the difference by the pronunciation of the word, even out of context. In my (East coast) accent it's more of a vowel length difference - 'sighed' is pronounced longer and slightly lower in the mouth than 'side'.

Leeneia's original question about lines that 'don't rhyme' anymore is to do with the Great Vowel Shift as DmcG linked to above.