Cellular chain complex ($C_{\ast }^{\mathrm{cell}}(X)$)

For a CW complex, $C_n^{\mathrm{cell}}(X)=H_n(X^n,X^{n-1})$ is freely generated by the oriented $n$-cells. Connecting maps of skeleton pairs define its differential.

The coefficient of an $(n-1)$-cell in the cellular boundary of an $n$-cell is the degree of the attaching map after collapsing the complement of that $(n-1)$-cell to obtain a map $S^{n-1}\to S^{n-1}$.