Handle
= Handle
{title2=$D^\lambda\times D^{d-\lambda}$}
An index-$\lambda$ <handle> in dimension $d$ is this product of <disks>, attached along $S^{\lambda-1}\times D^{d-\lambda}$. Its <core disk> is $D^\lambda\times\{0\}$, and its <belt sphere> is $\{0\}\times S^{d-\lambda-1}$. The complementary boundary piece is $D^\lambda\times S^{d-\lambda-1}$. Crossing a <critical point> in the <Morse handle-attachment theorem> produces precisely this local change.