I don't know why Thomastik doesn't include this as the default D-string in the Peter Infeld violin set - it is somewhat lower in tension than the Pi Silver D (10.1 lbs as opposed to 10.5 lbs, while the G string is 10.3 lbs) and the set seems to work much better with the lower tension; the whole instrument responds quicker and has much better resonance as a whole. The biggest improvement is on the G string, which has much greater depth and a "fatter" sound with the aluminum D string (while the silver D gives the sound a more "compressed" quality, and a more focused, laser-like tone at the expense of color and overtones). Comparing the D strings themselves, the aluminum D sounds slightly more brilliant and a tiny bit harsher under the ear than the silver D (which is "sweeter" sounding); the alum. D has much better carrying power, and produces significantly richer overtones. Less experienced players might be drawn towards the silver D because it is easier to produce a nicer sound under the ear, but seasoned professionals are likely to prefer the aluminum D. If you love Pi strings and haven't tried the aluminum D string yet, do yourself (and your violin, which will appreciate the slightly lower tension) a favor and give it a shot!