  The magnetic field-induced 3D ordered phase of the two-leg spin-ladder
Cu2(C5N2H12)2Cl4 has been probed through measurements of 1H NMR spectra and
1/T1 in the temperature range 70~mK - 1.2 K. The second order transition line
Tc(H) has been determined between Hc1 = 7.52 T and Hc2 = 13 T and varies as
(H-Hc1)^(2/3) close to Hc1. From the observation of anomalous shifts and a
crossover in 1/T1 above Tc, the mechanism of the 3D transition is argued to be
magnetoelastic, involving a displacement of the protons along the longitudinal
exchange (J//) path.
