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by the French gauge system. Improper interpretation of these standards may lead to confusion between the users and catheter manufacturers. The outer diameter (OD) and inner diameter (ID) are generally measured in inches or millimeters. The size of various tracheal tubes is usually expressed as ID, unless specified otherwise. Though the ID measurements may be constant, both OD and ID may show a difference of 2–4 mm depending on different manufacturers. For selecting an appropriate tracheal tube, the American society for testing and materials does not include any recommendations for OD measurement, as opposed to the recommendations available for ID measurements [1]. The OD and ID measurements of endotracheal tubes are summarized in Table 1. Armored or flexo-metallic endotracheal tubes have a slightly thicker wall. The difference between OD and a standard ID is greater in an armored tube as compared to a regular endotracheal tube (0.2–0.3 mm). The size of a double lumen tube is decided by the size of the bronchus; this corresponds to the external diameter equivalent of a French catheter gauge, defined as 3 times that of the ID, hence the OD here is more important. The difference in the ODs of Univent and double lumen tubes as compared to that of a regular endotracheal tube is 2–3 mm. In a study published by Al-Mazrou et al. [2], the internal tracheal diameter (ITD) was measured at the level of the cricoid, using magnetic resonance images, and compared to the OD of a utilized endotracheal tube (ETT). The OD of the best fit ETT was less than the ITD measured at the level of the cricoid by 0.1–1.7 mm, and it was concluded that the correlation of OD of the best fit ETT was strongerirrespective of weight or height. In another study by Bernet et al. [3], it was observed that the external diameter of pediatric tracheal tube cuffs could easily expand in vitro, to produce ODs of more than twice the age-corresponding ITD, when over-inflated with air. It has been observed that if the ID is too small, it may result in a smaller OD, which increases the cuff pressure required to create a seal in the trachea. A smaller ID tube also increases the Letter to the Editor