building block a block of material used in construction work
arch (architecture) a masonry construction (usually curved) for spanning an opening and supporting the weight above it
keystone -
a central cohesive source of support and stability; "faith is his anchor"; "the keystone of campaign reform was the ban on soft money"; "he is the linchpin of this firm"
support a military operation (often involving new supplies of men and materiel) to strengthen a military force or aid in the performance of its mission; "they called for artillery support"