T12 is an ultra-high-permeability MnZn ferrite with an initial permeability (μi) of 12000 — the highest in this catalogue — a saturation flux density (Bs) of 400 mT at 25 °C (210 mT at 100 °C) and a Curie temperature above 110 °C. The very high μi minimises the number of turns for a given inductance, so a common-mode choke achieves the highest common-mode impedance and the lowest DC resistance in the smallest volume; the relative loss factor at 100 kHz is below 15×10⁻⁶ and the hysteresis constant ηB 0.5×10⁻⁶/mT, giving low distortion and insertion loss in the signal path. Its limitation is the temperature range: with a Curie temperature of 110 °C and 210 mT at 100 °C it is intended for room- to moderate-temperature, low-flux signal and filter applications, not for DC bias or large flux swings. Typical applications are mains-input common-mode chokes, wideband and xDSL/Ethernet signal transformers, EMI filters and low-frequency current transformers. Within the high-permeability family, T10 (μi 10000, Br 60 mT) is recommended where a higher operating temperature and lower remanence are required, T05/T06 for general applications balancing temperature capability and Bs, and T07 as the compromise between μi and temperature stability.