  We prove Russo-Seymour-Welsh-type uniform bounds on crossing probabilities
for the FK Ising model at criticality, independent of the boundary conditions.
Our proof relies mainly on Smirnov's fermionic observable for the FK Ising
model, which allows us to get precise estimates on boundary connection
probabilities. It remains purely discrete, in particular we do not make use of
any continuum limit, and it can be used to derive directly several noteworthy
results - some new and some not - among which the fact that there is no
spontaneous magnetization at criticality, tightness properties for the
interfaces, and the existence of several critical exponents, in particular the
half-plane one-arm exponent.
