What is "tas-de-charge"

Tas-de-charge is a French language term in architecture, for which there is no equivalent in English, given to the lower courses of ribs of a Gothic vault, which are laid in horizontal courses and bonded into the wall, forming a solid mass; this helps bond the ribs, vault and walls together. They generally rise about one-third of the height of the vault, and as they project forwards they lessen the span to be vaulted over.