The Teredo auto-tunnelling protocol allows IPv6 hosts behind IPv4 NATs to communicate with other IPv6 hosts. It is enabled by default on Windows Vista and Windows 7. But Windows clients are self-constrained: if their only IPv6 access is Teredo, they are unable to resolve host names to IPv6 addresses. We use web-based measurements to investigate the (latent) Teredo capability of Internet clients, and the delay introduced by Teredo. We compare this with native IPv6 and 6to4 tunnelling capability and delay. We ﬁnd that only 6-7% of connections are from fully IPv6-capable clients, but an additional 15-16% of connections are from clients that would be IPv6-capable if Windows Teredo was not constrained. However, Teredo increases the median latency to fetch objects by 1-1.5 seconds compared to IPv4 or native IPv6, even with an optimally located Teredo relay. Furthermore, in many cases Teredo fails to establish a tunnel.