Rapid development is the opening principle of mobilizing all pieces to active squares as early as possible, before engaging in tactical or strategic operations.
Every opening move should ideally bring a new piece to a useful square. A knight on f3 or c3, a bishop placed on an open diagonal, a completed castling move — these actions build a concrete advantage. Conversely, moving the same piece twice or playing unnecessary pawn moves leaves pieces idle on their starting squares, creating a development lag that the opponent can exploit.
In practice, a straightforward rule guides the opening phase: develop a different piece each turn for the first six to ten moves, avoid moving the same piece twice unless absolutely necessary, and do not bring the queen out too early (it can be chased away, losing valuable tempos). The player who completes development first typically seizes the initiative — launching attacks while the opponent is still organizing their forces.