Mr. J.G. Carr and Sq.-Leader D. Scott, R.A.F., have devoted sections two, three and four of a total of 21, and a few other remarks here and there in their excellent paper, ‘The testing of airborne inertial navigation systems’ (Journal, 20, 405), to the above title as an exposition ‘first necessary’ to their main purpose, limiting themselves to ‘the types of system of which we have practical experience’. They do not otherwise identify these types explicitly, but do include them among the ‘many ways of mechanizing this concept', viz., ‘The feature which is common to all inertial navigators is the measurement of acceleration of the vehicle by measuring [and in effect they add also “or metering”] the force required to constrain a proof-mass carried in the vehicle to move with it,’ and from this deducing velocity and position of the vehicle. This statement seems well put to me and quite sound, but in the fourth paragraph of Section 4, which includes ‘Fig. 1. A Schuler tuned inertial navigator’, they imply that at least their types, if not all inertial navigators, are based also on ‘Schuler tuning [so named] after the discoverer of the principle', and here I do not follow them. (I note in passing that the by-line of Schuler's 1923 paper, Physik. Zeitschr. XXIV, 344–50, spells his name without the umlaut used consistently by them.)