An n = 1 locked, or slowly rotating, mode has been observed in most pulses prior to JET disruptions. However, a small fraction of non-disruptive pulses has a locked mode which eventually vanishes without disruption. Hence, on JET the locked mode amplitude is routinely used as an indicator of unhealthy plasma. There are two threshold levels of locked mode: (i) if locked mode amplitude exceeds the “low” threshold level, the emergency pulse shutdown is activated; (ii) if the locked mode amplitude exceeds the “high” threshold level, the massive gas injection (MGI) is triggered to terminate the pulse because the plasma is at high risk of disruption, and so it is necessary to mitigate disruption damage to the vessel and the Plasma Facing Component (PFC). On JET, four vessel octant data of 18 pick-up coils and 14 saddle loops can be used to calculate the locked mode amplitude and phase. There coils measure tangential to vessel poloidal magnetic field (B_p) and saddles normal to vessel field (B_n). The n = 1 locked mode amplitude and phase are calculated either from a subset of the pick-up coils or the saddle loops located in the identical poloidal location. The locked mode is usually interpreted as a tearing mode destabilized when rotation drops. For the monotonic plasma current profile, the O-point of the tearing mode island corresponds to a reduction of the measured tangential poloidal field. ECE data is presented to illustrate the large amount of data obtained during 2011-16 JET-ILW operation. The O-point of the locked mode has a preferred toroidal phase dwelling mainly in octants 4 and 5, which manifest in the machine toroidal asymmetries. The fact the locked mode exists for a long time prior to disruptions suggests other physics could be involved in finally triggering the disruption.