From: Rabbi Moshe Sofer
To: Rabbi Yosef Zalman
A student raises questions about a contradiction in the Ran regarding minors reciting berachos, the Magen Avraham's definition of permanent knots in tzitzis, and the Magen Avraham's critique of the Itur's two-hole method for threading tzitzis as creating a kilayim problem. The Chasam Sofer resolves each issue, most notably defending the Itur's practice using the Rambam's view that one tzitzis thread is half wool and half linen, producing only a single insufficient stitch of sha'atnez.
Source: Shu"t Chasam Sofer OC §1