Forward Referencing and Backward Referencing
Minimizing the Used Range
# Use one worksheet. It is faster to keep lookups and data on the same sheet.
# Whenever you can, SORT the data first (SORT is very fast), and use approximate match.
# When you must use an exact match lookup, restrict the range of cells to be scanned to a minimum. Use dynamic range names rather than referring to a very large number of rows or columns. Sometimes you can pre-calculate a lower-range limit and upper-range limit for the lookup.