Why is there a bulge at the top of my volumetric flask?

Sometimes volumetric flasks are designed with a bulge in the neck right below the stopper opening. What is the design intent of that bulge?


[![enter image description here](https://i.stack.imgur.com/t4uyu.png)](https://i.stack.imgur.com/t4uyu.png)




From *ASTM manual* [1, p. 204], emphasis mine:



> 
> Volumetric flasks should be diluted to just below the line, mixed thoroughly, then allowed to stand for 4 to 24 h to allow any heat from an acid-water reaction to dissipate. [...] Thorough mixing consists of at least 20 complete inversions of the volumetric flask, allowing enough time on each for the air bubble to completely traverse the length of the vessel. **Some borosilicate volumetric flasks contain a bulge in the neck above the line as an aid to proper mixing**.
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[Volumetric pipetts](https://en.wikipedia.org/wiki/Volumetric_pipette) also have a large bulge in the middle, but it serves a different purpose – to keep the $\rm length : volume$ ratio minimal and still maintain quick liquid collection with high precision.


### Reference


1. Dulski, T. R. A Manual for the Chemical Analysis of Metals; ASTM manual series; *ASTM: West Conshohocken, PA*, **1996**. ISBN 978-0-8031-2066-2.


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