If leading a spot card and wanting to give count, the lowest card was led from an odd number of cards and the third best was led from an even number of cards; if leading from a worthless holding and not wanting to give count, the highest card you could afford was led.
To provide count when leading a spot card, the lowest card should be led from an odd-numbered set, while the third-best card is led from an even-numbered set. If leading from a weak hand and not wanting to give count, use your highest available card.