Multi-crystalline silicon solidification is investigated by performing directional solidification experiments. Twinning phenomenon has been identified and observed in situ and in real time during the solidification using X-ray synchrotron imaging techniques: radiography and topography. The radiography observations give information on the formation, birth localized at the interface and evolution of the twins during solidification. The topography results give further information on the grain arrangement and on new grains in twinned position and grain growth competition. We have evidenced two twinning mechanisms: the first is the multiple twin formations during the growth of one grain. The second is the nucleation of a grain in twinned position at the bottom of a grain boundary groove.