Software provided under open source licenses is widely used, from forming high-profile stand-alone applications (e.g., Mozilla Firefox) to being embedded in commercial offerings (e.g., network routers). Despite the high frequency of use of open source licenses, there has been little work about whether software developers understand the open source licenses they use. To our knowledge, only one survey has been conducted, which focused on which licenses developers choose and when they encounter problems with licensing open source software. To help fill the gap of whether or not developers understand the open source licenses they use, we conducted a survey that posed development scenarios involving three popular open source licenses (GNU GPL 3.0, GNU LGPL 3.0 and MPL 2.0) both alone and in combination. The 375 respondents to the survey, who were largely developers, gave answers consistent with those of a legal expert's opinion in 62% of 42 cases. Although developers clearly understood cases involving one license, they struggled when multiple licenses were involved. An analysis of the quantitative and qualitative results of the study indicate a need for tool support to help guide developers in understanding this critical information attached to software components.