Stretching a string increases its tension. However, as soon as it is stretched, it starts to relax at the same time. You increase a string’s tension while stringing by pulling it to reference tension faster than it is simultaneously losing tension. As soon as you clamp off, the string continues to lose tension forever, but at a decreasing rate. Most of the tension is lost in the first few minutes after clamping (10–25%). This rapid loss stabilizes after a few hours, but another 5–10% will be lost on the first several ball impacts.
Every second of time and every impact decreases tension a little. Different strings hold tension better in each circumstance. The combined loss (all that matters in the end) is what we have reported on here.
Gut is the best at retaining tension, and typically, polyesters and aramids are the worst.