?have a synthetic nylon core that has all the wonderful attributes of a gut core, but will last much longer and will not break down like gut strings.? The C,G and D strings are all silver wrapped, while the A is aluminum. While the (#141's) come in the usual 3 gauges, of weich (light), medium or heavy, the most popular weight and tension are the mediums.? They are warm and very responsive and also well known for uniformity of sound response and evenness across all the strings.??