Part: Part Four - Pot Odds & Hand Analysis

Not all outs are clean

Pre-flop
Blinds 100 / 200Pot 900 (4.5 BB)BTNQ♥9♥● 20,000 (100 BB)YOUto actMP● 20,000 (100 BB)Openerraises 600 (3 BB)SB● 20,000 (100 BB)posts 100 (0.5 BB)BB● 20,000 (100 BB)posts 200 (1 BB)D

It folds to you on the button with Q♥9♥ after a middle-position raise.

Opener makes it 600 (3 BB); you hold Q♥9♥ on the button. Best?

WhyCall. A suited, connected broadway in position is a reasonable call - just be ready to read the board carefully when you flop a draw.
What happensYou call; the blinds fold.  Pot: 1,500 (7.5 BB).
Flop
Heads-upPot 1,500 (7.5 BB)J♠10♠4♦BTNQ♥9♥● 20,000 (100 BB)YOUopen-ender, but two outs are 'tainted'MP● 20,000 (100 BB)Openerbets 900 (4.5 BB)D

Flop: J♠ T♠ 4♦ - you flop an open-ended straight draw (any K or 8). But the board is two-spade, so the K♠ and 8♠ complete a flush for anyone on a spade draw.

Eight 'outs', but two of them (K♠, 8♠) can hand someone a flush. Your effective outs?

WhyAbout six. Two of your eight straight cards also complete a flush, so against a likely spade draw they can make your straight a loser - you discount them. Six clean outs is only ~24% by the river (rule of 4), and facing a bet and possible re-raise on a wet board, this is now a marginal-to-losing call.
What happensRecognizing the taint, you fold rather than overpay a discounted draw.  You fold.
You started with eight outs but two of them could make a second-best hand, so your real equity was lower than it looked - and the call you'd have made at eight outs becomes a fold at six. Count clean outs, not gross ones.

Discount outs that can make you a second-best hand (cards that also complete a flush or pair the board) - effective outs, not gross outs, decide the call.