The string pattern is written as mains × crosses — 16×19 means sixteen strings running up the frame and nineteen across it. A denser bed (18×20) has more strings in the same head, so each one moves less. An open bed (16×19) lets the mains slide sideways and snap back, which is where topspin comes from.
That is the theory. Our catalogue lets us put numbers on it. Across the 191 frames we hold in these two patterns, the 14 with an 18×20 average 9.5 on control, 7.5 on spin and 7.5 on power. The 177 with a 16×19 average 8.3 on control, 8.4 on spin and 8.4 on power. One point of control against one point of spin, near enough exactly.
So the pattern is not a quality question, it is a trade. A dense pattern is for a player who already generates their own spin and wants the ball to go precisely where it was aimed. An open pattern is for a player who wants the string bed to add shape to the ball for them.
Two practical notes. First, an open pattern eats strings: the mains saw against the crosses every time they snap back, which is why a 16×19 breaks a polyester sooner than an 18×20 does. Second, the same frame in both patterns is not the same racquet — the 18×20 version usually feels stiffer through the hand even when the layup is identical, because a denser bed deflects less.
If you are choosing between the two versions of one model, our comparison table puts them side by side on all six criteria, and the tension calculator will propose a different tension for each — a dense pattern normally wants a kilo or two less to keep the same feel.