As the article says, it is impossible to make the back ends of the white keys equal in width so piano makers have to compromise. The difference is about a milli

As the article says, it is impossible to make the back ends of the white keys equal in width so piano makers have to compromise. The difference is about a millimetre so many pianists may never have noticed. The naive approach of placing each black key at the midpoint of its adjacent white keys makes B, C, E and F quite wide at the base, but it is harder to fit a finger between the black keys to play D, G or A, which can be necessary when the hand has to stretch or play both black and white keys. On a real piano, therefore, the C# and D# keys and the F# and A# keys are offset a little from the midpoints of their adjacent white keys. G# is the only black key that is actually at the midpoint of its adjacent white keys. In the photograph of the Yamaha piano, you can see that the cutouts on the D key are symmetrical but less than half the width of a black key because the C# and D# keys are offset. Looking at the G key, the right cutout, at half the width of a black key, is deeper than the cutouts on the D key, so to compensate the left cutout is less deep than the cutouts on the D key. As a consequence, the cutout on the F key is deeper than the cutout on the E key, so the E key is wider at the back than the F key. Similarly, the C key is wider at the back than the B key. As described in the article, on some keyboards just the C and E keys are wider at the back, with D the same width at the back as F, G, A and B. More often, however, the C# and D# keys are placed a little further out to spread the extra width equally between the C, D and E keys.