In chess, the minor pieces are the knight and the bishop — as opposed to the major pieces, the rook and the queen. Both are conventionally valued at roughly 3 points on the standard material scale, equivalent to three pawns each.
This approximate equality between knight and bishop is one of the first concepts to internalize: the two pieces are worth about the same, yet their actual strength varies greatly depending on the pawn structure. The bishop thrives in open positions, where its long diagonals are unobstructed; the knight excels in closed positions, where its L-shaped jumps allow it to leapfrog over the pawn chains that stifle other pieces. A well-placed knight sitting on a central outpost can prove more powerful than a bishop locked behind its own pawns.
In practice, always assess which of your minor pieces is more active before agreeing to a trade. Exchanging your well-centralized knight for your opponent’s passive bishop is almost always a good deal. Conversely, avoid giving up an active minor piece for a passive one without clear compensation.