Assist features are an essential component of developing an integrated imaging system that can produce near resolution limit sized semiconductor features. Of the features in use today, binary and phase-shifted scattering and anti- scattering bars are proving critical to the success of making current and next generation devices in production. These features modify the diffraction pattern of isolated lines to give them some dense line imaging attributes thus making it easier to overlap and enhance the process window for these features. They have also been shown to reduce the mask error enhancement factor and the effect of aberrations. This paper provides an analytic description of these assist features and uses this description to explain how and why these features work. Working in frequency space, it also demonstrates how to use these assist feature with illuminator and phase-shift image enhancement techniques to build a robust semiconductor imaging process.