Let $G$ be a group. Then $G$ has a $BN$ -pair or a Tits system if the following conditions hold:

1. $B$ and $N$ are subgroups of $G$ such that $G=<B,N>$ .
2. $B\cap N =T \triangleleft N$ and $N/T=W$ is a group generated by a set $S$ .
3. $sBw \subseteq BwB \cup BswB$ for all $s\in S$ and $w\in W$ .
4. $sBs^{-1} \not\subseteq B$ for all $s\in S$ .

Where $BwB$ is a double coset with respect to $B$ . It can be proven that $S$ is in fact made up of elements of order 2, and that $W$ is a Coxeter group.

Example: Let $G=GL_n(\mathbb{K})$ where $\mathbb{K}$ is some field. Then, if we let $B$ be the subgroup of upper triangular matrices and $N$ be the subgroup of monomial matrices (i.e. matrices having one nonzero entry in each row and each column, or more precisely the stabilizer of the lines $\{[e_1],...,[e_n]\}$ ). Then, it can be shown that $B$ and $N$ generate $G$ and that $T$ is the subgroup of diagonal matrices. In turn, it follows that $W$ in this case is isomorphic to the symmetric group on $n$ letters, $S_n$ .

For more, consult chapter 5 in the book Buildings, by Kenneth Brown