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'''Depth''' is the height or ''nominal'' depth in [[Ply|plies]] between the [[Root|root]] and so called [[Horizon Node|horizon nodes]] (depth 0), where a heuristic value is assigned to. Thus, depth is the number of half moves the search ''nominally'' looks ahead.
Despite [[Quiescence Search|quiescence search]], where usually winning captures and even some checks are tried at or behind the search horizon, until positions become sufficiently quite, [[Selectivity|selectivity]] of modern chess programs, caused by [[Extensions|extensions]], [[Pruning|pruning]] and [[Reductions|reductions]], notably [[Check Extensions|check extensions]], [[Null Move Pruning|nNMPNMP]] and [[Late Move Reductions|LMR]], leads to bushy, non-uniform [[Search Tree|trees]] where some branches are searched deeper than nominal, but others shallower. A [[Depth Reduction R|depth reduction R]] of multiple plies is often performed in forward pruning techniques like [[Null Move Pruning|null move pruning]] and [[Multi-Cut|multi-cut]].
=Draft versus Ply-Index=
