Apropos of the virtue of negative subscripts. data aliasing situation Not if all references are relative to the offset pointer. The following example is silly, but I recently used exactly this trick to simplify calculations on a first-quadrant (x,y) grid by supplying "out-of-bounds" zeroes at (x,-2), (x,-1) and (-1,y). It was much cleaner to access the zeroes like ordinary elements than to provide special code to handle the boundary conditions. /* Fill an N-element array with Fibonacci numbers, f(i), where 0<=i<N. The recursion accesses a zero "out of bounds" at i=-1 */ const int N = 100; int base[N+1]; #define fib(i) base[(i)+1] void fill() { int i; fib(0) = 1; for(i=1; i<N; i++) fib(i) = fib(i-2) + fib(i-1); } Doug