Nails are sold by their size (型号, xínghào). Size depends on the length of your middle finger’s pad, measured from the tip to your first knuckle. Your goal is to have nails that cover most of your pad and extend 3-5mm beyond the tip of your finger. Sizes are recommended for certain ages or certain heights. In general:
Small (小号, xiǎo hào):
Who: children below the age of 9 or under 1.3m tall (51 inches, 4’3”)
Nail Lengths: Finger:22-23mm Thumb: 24-25mm
Medium (中号, zhōng hào):
Who: adolescents between 10-15 or between 1.3 and 1.6m tall (51-63 inches)
Nail Lengths: 24-26mm Thumb:27-28mm
Large (大号, dà hào):
Who: people over the age of 16 or taller than 1.6m (63 inches, 5’3”)
Nail Lengths: 27-29mm Thumb: 30-31mm
Thumb nails are measured by the furthest two points on the nail. Two nails can have the same thumb length but vastly different reach and design.
The next big difference between nail sets is the curvature of their surfaces. You will find these and the indent below called out under ‘‘规格’’ (guīgé , specification) alongside the indents, below.
Flat, beginner nails. - Both sides are flat. These are almost always thin and considered insufficient for advanced techniques. Often found taped to cards. This is not often labeled, it’s obvious.
单弧 (dān hú) - “Single Arc” - one side is flat while the other is curved. The curved surface is what strikes the strings; the flat side is what touches the skin. The finger side can be flat or it can be indented. These are considered easier to use than Double Arc nails but are not as effective at some techniques.
双弧 (shuāng hú) - “Double Arc” - the nails are curved on both sides. This nail style is said to provide the best playing experience across all techniques but is harder to control. It allows for focused force transmission between the string and the finger. Proper taping technique is important as these nails are less stable on the finger when taped improperly.
Nails can be shaped to better fit the finger. Each shape comes with benefits and costs.
Flat - Easy to make and cheaper to buy and store, but offer less control. They are prone to shifting on the finger. Flat sides are often found on beginner’s nails and 单弧 (dān hú, Single Arc) nails.
凹槽 (āo cáo) - “Concave Groove”. The string side is convex while the finger side has an indent for your finger’s pad. It’s great for beginners because it is very stable and comfortable. Some people warn it can cause fingers to sweat in the summer.
双槽 (shuāng cáo) - “Double Groove” The string side is curved while the finer side has two grooves cut in the surface. This is supposed to increase stability but varies person to person. Some people may find the ridge presses uncomfortably into the finger while it may be just fine for others.
全弧 (quán hú) - “Full Arc” The entire finger side of the nail is concave rather than just an indent. This is apparently a newer style for people with wider fingers. It’s not pictured as the nail set I have is translucent and difficult to photograph.
Thicker nails are less flexible, emphasize lower pitches, and are less prone to breaking. They are however a bit heavier and can be difficult for smaller children or those with weaker fingers. Thin nails require less strength and emphasize higher frequencies but their flexibility can make some techniques difficult.
Thickness is measured in millimeters and can be anywhere from less than 1.5mm to more than 3.5mm. Flat nails tend to be on the thin side while tapered nails tend to be thicker. From one recommendation:
Age < 7 years, grades 1-3: 1.5mm
Grades 4-5: 2.0mm
Grades 7-10: 2.5mm
High level students: 2.5mm to 3.0mm
Beyond 3.0mm: players who specifically need the extra thickness for their techniques.
Note: It is unclear if “grade” refers to guzheng playing ability (1-10) or the year of school.
There is one other factor: The angles of the nail. Now, there is no standard measuring system that I know of but the end result is the same: what angle does the thumb nail make with the rest of the finger? How sharp is the angle on the point of the nail? How quickly does the thickness of the nail decrease to nothing? There is a general practice but individual nails vary. If you find yourself excelling with one set of nails and struggling with another, you’re not crazy. Small differences do matter.
On the thumb angle there is a newer style that is gaining popularity called 半月型 (bànyuè xíng ) “Half-moon type”. The thumb is so curved it looks like a crescent moon. No word on how that affects playing or their sound.