I think the key ’missing chord’ that so many have been searching for is the jazzy and appropriately ’blue’ Em7b5/Bb used in the second line of the chorus the bridge and then it’s the penultimate chord of the song. Em7b5/Bb   x12030 or x1203x In the ’boasting of loving’ section I think Costello strums F#7 (note NOT F#7 minor) throughout - I’ve added alternate chords in brackets that follow the bass line and harmonise but I don’t think Costello plays this. [Intro stays on D] Now it’s just you and me my blue friend And you say that it’s you that she’s thinking of And our affair must end But if it’s you that she’s thinking of I think my broken heart might mend Now it’s my turn to talk and your turn to think Your turn to buy and my turn to drink Your turn to cry and my turn to sink Down in the Blue Chair Down in the Blue Chair Now I’ve made up my mind I’ve made my mistake And I know that she cries for you When she’s barely awake Well she’s going to bend your mind Well I hope it don’t break [Repeat chorus] Down in the Blue Chair We can watch our troubles rise Like smoke into the air And drift up to the ceiling Down in the Blue Chair You can feel just like a boy or a man The next minute you can find yourself kneeling Down in the Blue Chair           G#m7b5      A6         G#m7b5 They’re boasting of loving the daylights       F#           G#m7b5      Bm right out of her in the small hours Down in the Blue Chair You say that your love lasts forever when you know the night is just hours     uh ho ho ho ho ho And still I want her right now Not any minute hour or day And wherever she is tonight I want her anyway I suppose she never said to you You were just in the way Now it’s my turn to talk and your turn to think Your turn to buy and my turn to drink Your turn to cry and my turn to sink Down in the Blue Chair Down in the Blue Chair Down in the Blue Chair Down in the Blue Chair   Down in the Blue Chair Down in the Blue Chair Down in the Blue Chair   Down in the blue Blue becomes you Down in the Blue Chair