A number of software applications for the practice of the singing voice have been introduced the last decades, but all of them are limited to equal tempered scales. In this work, we present the design and development of FONASKEIN, a novel modular interactive software application for the practice of singing voice in real time and with visual feedback for both equal and non-equal tempered scales. Details of the Graphical User Interface of FONASKEIN are given, along with its architecture. The evaluation results of FONASKEIN in a pilot experiment with eight participants and with four songs in various musical scales showed its positive effect in practice of the singing voice in all cases.