Where Edge Leaks Away

Every chapter so far has assumed a clean number: you enter at exactly the price you planned, and exit at exactly the price your stop specified. Real markets don't work that way, and the gap between the plan and the fill has a name — slippage — and it's this chapter's entire subject.

Look at the figure: an order book, the real structure standing behind every price you see on screen. At ₹500.00, there are only 300 shares available. Your order wants 500. The moment your market order arrives wanting more than the book can offer at the best price, it doesn't stop — it eats upward through the next level, and the next, chasing whatever quantity remains, at progressively worse prices, until your full order is filled.

Your average fill price ends up worse than the price you saw when you clicked. Not because of a fee, not because of a broker charge — because the order book, at that instant, simply didn't have enough patience (enough resting orders) to absorb your size at the price you wanted. This is slippage, and it is a structural feature of markets, not a malfunction.

Two forces decide how much slippage you'll pay on any given trade, and both are worth knowing by name:

The bid-ask spread — the permanent, ever-present gap between the highest price a buyer is currently offering (the bid) and the lowest price a seller is currently asking (the ask). Every market order effectively crosses this spread, buying at the ask and selling at the bid — a small, constant cost that exists even in perfectly calm, deep markets. In a liquid large-cap stock, the spread might be a few paise. In a thin option strike or an illiquid small-cap, it can be a meaningful percentage of the price itself — a toll you pay on every single entry and exit, win or lose.

Liquidity depth — how much genuine buying and selling interest actually sits behind the visible best price. A liquid index future or a large-cap stock might show thousands of shares resting at each price level; a thin option strike or a micro-cap might show only a handful. The figure's order book, with its thinning levels, is exactly this: size that would barely register in Nifty futures can move a small-cap's price meaningfully, purely because the depth to absorb it simply isn't there.

Now the practical toolkit — the order types that let you control how much of this cost you pay, matched to the situation:

Market order: "fill me now, at whatever price it takes." Guarantees the fill, not the price. Correct when speed matters more than precision — exiting a position that's actively working against you, where the cost of a few extra rupees of slippage is trivial next to the cost of not exiting at all.

Limit order: "fill me only at this price or better." Guarantees the price, not the fill — in a fast-moving or thin market, a limit order can simply never execute, leaving you holding a position you meant to have exited. Correct when price matters more than certainty of fill — entering a position where chasing a worse price defeats the trade's own logic.

SL and SL-M — you met these properly in Chapter 3: SL-limit guarantees price on trigger, risking a failed fill in a fast market; SL-market guarantees the exit, risking a worse fill. The same market-versus-limit trade-off, applied specifically to your stop.

GTT (Good Till Triggered) orders let you place a standing instruction — buy or sell if price reaches a level — that persists across sessions without you having to watch the screen or re-enter it daily. Useful for structural stops and pre-planned entries that may take days to trigger; it doesn't change the market vs limit trade-off underneath, but it removes the human failure mode of forgetting to place the order at all.

One India-specific structural danger deserves its own warning, because it combines two of this chapter's ideas at their most extreme: thin option strikes, especially far from the money or near expiry. These can show wafer-thin depth at every level, spreads that are a genuinely large percentage of the premium, and — during periods of high volatility — the risk of circuit limits, where an instrument's price is prevented from moving further in one direction, leaving you unable to exit at any price at all until the circuit resets or the underlying moves back into range. A strategy that looks perfectly profitable on a backtest using clean closing prices can be quietly unprofitable in practice, once real spreads and real slippage on real strikes are subtracted — precisely the frictionless-world lie the Algorithmic Trading school warned you about, now costing you money one trade at a time instead of all at once.

The size of this leak is easy to underestimate because no single trade makes it dramatic. A few paise here, a slightly worse fill there — it doesn't announce itself as a loss the way a blown stop does. It simply, quietly, subtracts a small amount from every single trade's expectancy (Chapter 4's R-multiples, eroding at the edges before you even see them), trade after trade, for as long as you trade. The strategies that survive contact with real markets are the ones that accounted for this leak from the start — not the ones that discovered it by watching a beautiful backtest fail to reproduce itself live.

Position sized correctly, stop placed by structure, leverage respected, execution costed honestly. One box remains — and it's the one most traders never open until the day every position they own falls at once.