Standard taper · 1:10
Ground glass joints, and what the numbers mean
24/40 and 24/29 look almost the same and are not the same. This guide explains the notation, shows which joints mate with which, and covers greasing, clipping and freeing a seized joint.
Notation
Two numbers, two different things
A standard taper joint is written as two numbers separated by a slash. They describe completely different measurements, which is why swapping one for another rarely works.
The first number is the outside diameter, in millimetres, of the male cone at its widest point — the top of the ground zone. This is the number that decides whether two pieces will physically mate at all.
The second number is the length of the ground surface, in millimetres. It decides how much sealing area you get, and how far the inner joint reaches into the outer one.
Every standard taper joint shares the same cone angle: a 1:10 taper, meaning the diameter shrinks by 1 mm for every 10 mm of length. That shared angle is what makes the system work — and it is why a 24/40 cone slides into a 24/29 socket even though the pairing is not a standard one.
Interactive
Will these two joints fit?
Pick an inner cone and an outer socket. The drawing is scaled to the real dimensions of whatever you choose.
Verdict
Drawn to scale. Both halves use the same 1:10 taper.
Reference
Common standard taper sizes
Two families are in circulation. North American glassware is usually ASTM; European and most internationally traded glassware is ISO/DIN, where the size is often written with an NS prefix — NS 29/32 is the same thing as 29/32.
ISO / DIN series
| Size | Wide end | Narrow end |
|---|---|---|
10/19 | 10.0 mm | 8.1 mm |
14/23 | 14.5 mm | 12.2 mm |
19/26 | 18.8 mm | 16.2 mm |
24/29 | 24.0 mm | 21.1 mm |
29/32 | 29.2 mm | 26.0 mm |
34/35 | 34.5 mm | 31.0 mm |
45/40 | 45.0 mm | 41.0 mm |
24/29 is the most widely used size internationally.
ASTM series
| Size | Wide end | Narrow end |
|---|---|---|
14/20 | 14.5 mm | 12.5 mm |
19/22 | 18.8 mm | 16.6 mm |
24/40 | 24.0 mm | 20.0 mm |
29/42 | 29.2 mm | 25.0 mm |
34/45 | 34.5 mm | 30.0 mm |
45/50 | 45.0 mm | 40.0 mm |
24/40 is the standard in most American laboratories.
The trap. 24/40 and 24/29 both start at 24 mm, so a 24/40 cone will slide into a 24/29 socket. But the cone is 11 mm longer than the socket is deep, so it protrudes past the ground zone and only part of the surface actually seals. It may hold for an open reflux; it should not be trusted under vacuum or with anything hazardous. Use a proper adapter instead.
Beyond the cone
Other joint types you will meet
Ball and socket
Written like 18/9 or 28/15: the first number is the outside diameter of the ball at its base, the second is the bore through the middle. They must be clamped, but they tolerate misalignment — invaluable for heavy apparatus or long runs of glass that will never line up perfectly.
O-ring joints
A flat or bevelled glass face sealed by an elastomer O-ring under a clamp. No grease, no seizing, and they take angular misalignment. The limit is the O-ring itself: check its chemical and temperature rating, not the glass.
GL screw threads
Written GL14, GL18, GL25, GL45, where the number is the thread outside diameter in millimetres. Sealed with a cap and a PTFE-faced septum or silicone liner. Common on bottles, and on tubing take-offs where a ground joint would be overkill.
In use
Grease, clips and seized joints
Grease the top third, thinly. Apply a narrow band around the upper part of the cone and rotate the joint together until the film goes clear. Grease on the lower end does nothing except drip into your flask. If you cannot have grease near the chemistry at all, use a PTFE sleeve instead.
Choosing a clip
| Type | Limit to know |
|---|---|
| Polyacetal (Keck) clips | Colour-coded by size and cheap, but the plastic begins to soften around 140 °C and degrades quickly in corrosive vapours. |
| PTFE clips | Far better thermal and chemical resistance, and more expensive. Overheating PTFE releases hazardous decomposition products. |
| Stainless steel springs and clamps | Handle the full temperature range of the glass, but can corrode in aggressive atmospheres. |
When a joint seizes
Joints freeze for three main reasons: solvent has washed the grease out, base has attacked and partly dissolved the ground surfaces, or the two halves were at different temperatures when assembled. Work through the gentle options first.
- Work solvent inApply solvent at the seam and rock the inner joint gently rather than twisting hard.
- Exploit thermal expansionWarm the outer joint, or cool the inner one, so the two move apart rather than together. Never heat both.
- Apply leverage carefullyUse joint separator pliers with the apparatus supported. Never lever against an unsupported flask.
Stop before force. A seized joint that will not yield to solvent and gentle thermal cycling is a piece of glass about to break in your hands. Wrap it, set it aside, and treat it as a replacement.
Shop
Adapters, clips and jointed glassware
If two pieces will not mate, an adapter almost always exists. Synthware stocks reducing and expanding adapters across the standard taper range.