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Words with an obvious ɑː sound
are, aren't, arm, art, artwork, artist, chart, charter, part, parting, tart
far, farther, farthest, car, bar, barbecue, dark, star, start
fast, last, past, plaster, cast
father, rather, lather (but not bather or gather)
Words with a surprise ɑː sound
heart, hearth, aunt, laugh, laughter, after, afternoon, raft, craft, rafter, shaft