The term 'Musical Fourths' refers to a musical interval consisting of two notes that are four scale degrees apart. The interval is also known as a perfect fourth, and is one of the most commonly used intervals in music. The crossword solution for this term is FAS, which is an abbreviation for the Italian phrase 'fatto a sei', meaning 'made with six'. This phrase is used to describe the process of creating a perfect fourth interval by dividing the octave into six equal parts.