The Grammarphobia Blog: Why don’t “laughter” and “daughter” rhyme?
What’s very interesting about language is how they change over time. Words that used to rhyme no longer rhyme, and words that didn’t rhyme before now rhyme. This is the case with laughter and daughter. Sounds can change over time and affect different words differently, which is why great and seat don’t rhyme even though they both have the “ea” sound in them