球拍balance的問題

Balance is the equilibrium point of the racquet. As you say, 31.5cm / 9pt HL (Headlight) are the same, means the balance point is at 31.5cm measuring from the butt cap. For a standard 27" racquet is equal to 68.6cm, which the mid way is 34.3cm. So, 31.5cm HL is which the balance point is moving towards the butt cap, and where is 2.8cm from the mid point. For which 9pt is another way of expressing the balance point of a racquet, which 1pt is equal to 1/8 of an inch. So, in this case, 2.8cm HL = 1.1in HL, which is 8.8pt HL around up to 9pt HL.

On your other question, which is the string pattern, 16x18, 18x20, are indicating the number of string on Mains and Crosses. This actually is giving you a rought idea on string density. String density is directly affecting the hardenness of the string bed, and it is much more important than the tension pull on the string itself only. For instance, if you strung at 60lbs on a 16x18 racquet, compare to 60lbs on a 18x20 racquet, which you will find the string bed is will give you at least 15% harder stringbed, means less power. But of course, the number of string will only give you an approximation, as the string density is not evenly distributed on a hoop. A typical example is on the new K90 Wilson racquet, which is sharing the same number of string as the n61 90, but the string density on the sweetspot area is much less, result to a bit more power.

Am I answered your question? And hope they help.