When Shardul Thakur was no-balled on the fourth day of the final Border-Gavaskar Trophy series Test at Brisbane, it prompted some onlookers to question whether it was a fair decision after Mitchell Starc went unpunished for a similar spell earlier in the Test.

Shardul was no-balled after bowling four consecutive short balls to Australia’s ninth wicket pair of Pat Cummins and Nathan Lyon. The first was pulled by Lyon for a single, the second ducked under by Cummins, the third missed as Cummins attempted to hook and the fourth left alone.

Although Shardul didn’t overstep, there are a number of possible reasons for a no ball to be given. According to Clause 41.6 of the ICC’s Test match playing conditions, a no ball can be given for the ‘Bowling of dangerous and unfair short pitched deliveries.’

As per the regulations, bowlers are limited to two fast short-pitched deliveries per over, with ‘fast short-pitched deliveries’ defined as balls that pass, or would have passed “above the shoulder height of the striker standing upright at the popping crease.” As per clause 41.6.2, If a bowler delivers more than two of such deliveries in an over, the umpire is expected to call a no ball.

This is the most obvious explanation for the Shardul no ball call. However, it’s not clear whether or not this was the exact reason for the no ball. The regulations also stipulate that an umpire can also call a no ball if they deem the bowling to be dangerous and unfair. Clause 41.6.1 reads: “Notwithstanding clause 41.6.2, the bowling of short pitched deliveries is dangerous if the bowler’s end umpire considers that, taking into consideration the skill of the striker, by their speed, length, height and direction they are likely to inflict physical injury on him. The fact that the striker is wearing protective equipment shall be disregarded.”

If the latter is the reason for the no ball, India may feel aggrieved not to be awarded runs for no balls in their own innings. The first five deliveries Mitchell Starc, widely regarded as the world’s fastest bowler, bowled to T Natarajan – a Test debutant with a first-class batting average of two – were all short, yet Starc was not called for a no ball. It is not clear which of these deliveries, if any, were called as passing “above the shoulder height of the striker standing upright at the popping crease.”

As pointed out by ESPNcricinfo journalist Daniel Brettig, Starc also went unpunished in a similar spell in the Boxing Day Test where he delivered 24 short-pitched balls during a five over spell at India’s tailenders. On that occasion, Starc was bowling at all-rounder Ravichandran Ashwin and specialist quick Umesh Yadav.