 Binary linear code

A linear code over the two-element finite field. Its dimension $r$ gives $2^r$ codewords. Its weight enumerator in the convention $W_C(s,t)={\sum }_{c\in C}{s}^{w(c)}{t}^{n-w(c)}$ is symmetric in $s,t$ exactly when the all-ones word belongs to $C$: adding that word complements every bit and replaces weight $w$ by $n-w$. Conversely symmetry makes the coefficient of $s^n$ equal the coefficient of $t^n$, which is one.