The BLAS (Basic Linear Algebra Subprograms) are high quality "building
block" routines for performing basic vector and matrix
operations. Level 1 BLAS do vector-vector operations, Level 2 BLAS do
matrix-vector operations, and Level 3 BLAS do matrix-matrix
operations. Because the BLAS are efficient, portable, and widely
available, they're commonly used in the development of high quality
linear algebra software, LINPACK and LAPACK for example.