phase diagram graph explanation

At point a3, I know the ratio of the liquid phase to the solid phase, but I don't get the highlighted statement which says b3 gives the composition. I thought a3 already gave the composition? Do I need to apply another lever rule where b3 is the center? If so, where would the endpoints be?


The statement refers to the composition of the liquid phase, which is simply the x-value of $b_3$.

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    $\begingroup$ To more fully flesh it out: the overall composition remains at (a2,a3,...). However, the composition of the liquid is read off the liquidus curve, so is b2, while the composition of the solid is read off the solidus curve. For this diagram there is no mutual solid solubility in the solid, so the solid is pure B. $\endgroup$ – Jon Custer Nov 22 '17 at 16:51

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