Padstones are the first and last brick or stone of an Arch which are seated on the Spring Line. Padstones are used in buildings to stop the "cheese were" effect of steel cutting the brick or the blocks in the building.
[Co] A flat slab of stone placed on the ground to provide a support for a wooden upright forming part of the structure of a timber-framed building. A group of padstones is often the only archaeological trace remaining of such buildings. Originally www.answers.com