Bonisagus, Guernicus, Mercere, and Tremere Houses are the True Lineages, which means that each magus was taught by a magus of that House, and that master-apprentice relationship goes back to their founders.
Bonisagus, Guernicus, Mercere, and Tremere Houses are the True Lineages. Bonisagus, Guernicus, Mercere, and Tremere Houses are that master. Each magus was taught by a magus of that House. The True Lineages means this. Apprentice relationship goes back to their founders.