While the "Duplex Theory" of sound localization is often interpreted such that low-frequency sounds are localized mainly based on interaural phase differences, and high-frequency sounds based on interaural level differences, some studies have shown an interaction of low-frequency interaural phase and level differences. Using a psychoacoustic lateralization experiment, the present study demonstrates that small interaural level differences are indeed effective in resolving lateralization in the ambiguous range of interaural phase differences at all tested frequencies. These ambiguities occur in free-field conditions at frequencies above about 500 Hz, which is shown by analyzing the magnitude of interaural differences as they occur in typical free-field scenarios. On that basis this study further concludes that naturally occurring interaural level differences on their own are sufficient to correctly attribute sound sources to left or right in many conditions, even at frequencies below 500 Hz. © 2018 The Author(s). Published by S. Hirzel Verlag · EAA. This is an open access article under the terms of the Creative Commons Attribution (CC BY 4.0) license (https://creativecommons.org/licenses/by/4.0/).