  A bramble in an undirected graph $G$ is a family of connected subgraphs of
$G$ such that for every two subgraphs $H_1$ and $H_2$ in the bramble either
$V(H_1) \cap V(H_2) \neq \emptyset$ or there is an edge of $G$ with one
endpoint in $V(H_1)$ and the second endpoint in $V(H_2)$. The order of the
bramble is the minimum size of a vertex set that intersects all elements of a
bramble.
  Brambles are objects dual to treewidth: As shown by Seymour and Thomas, the
maximum order of a bramble in an undirected graph $G$ equals one plus the
treewidth of $G$. However, as shown by Grohe and Marx, brambles of high order
may necessarily be of exponential size: In a constant-degree $n$-vertex
expander a bramble of order $\Omega(n^{1/2+\delta})$ requires size exponential
in $\Omega(n^{2\delta})$ for any fixed $\delta \in (0,\frac{1}{2}]$. On the
other hand, the combination of results of Grohe and Marx and Chekuri and
Chuzhoy shows that a graph of treewidth $k$ admits a bramble of order
$\widetilde{\Omega}(k^{1/2})$ and size $\widetilde{\mathcal{O}}(k^{3/2})$.
($\widetilde{\Omega}$ and $\widetilde{\mathcal{O}}$ hide polylogarithmic
factors and divisors, respectively.)
  In this note, we first sharpen the second bound by proving that every graph
$G$ of treewidth at least $k$ contains a bramble of order
$\widetilde{\Omega}(k^{1/2})$ and congestion $2$, i.e., every vertex of $G$ is
contained in at most two elements of the bramble (thus the bramble is of size
linear in its order). Second, we provide a tight upper bound for the lower
bound of Grohe and Marx: For every $\delta \in (0,\frac{1}{2}]$, every graph
$G$ of treewidth at least $k$ contains a bramble of order
$\widetilde{\Omega}(k^{1/2+\delta})$ and size
$2^{\widetilde{\mathcal{O}}(k^{2\delta})}$.
