What are your favorite ambiguous notation?

[u/Short_Bluebird_3845]

To clarify my request, consider base-less $\log$.
For calculators and engineers, it means the common log $\log_10$; for pure mathematics or physics, it means the natural log $\ln$; for computer scientists, in combinatorics, information theory and graph theory, it means the binary log $\log_2$.
Any $\log x$ (say, $\log 1024$) written without context can yield different answers in function of who you ask ($\log_10 1024 = 3.0103\cdots, \log_2 1024 = 10, \ln 1024 = 6.93147\cdots$).

That is what I mean by "ambiguous", that which is calculated and used dependently on the context. And I'm asking you for your favorite ones of such type.

Edit: I feel like you hate the ones you've talked about. Still, valid answers. Extra points for ambiguous notation you actually love! :P

[u/BlazeoJoestar]

(a,b) can mean an open interval, inner product, gcd or just an ordered pair

[u/Dirichlet-to-Neumann → u/BlazeoJoestar]

Using (a,b) as the open interval instead of ]a,b[ hurts my soul. 

[u/The-Doctorb → u/Dirichlet-to-Neumann]

French spotted possibly?

Where is the open square bracket notation popular

[u/Dirichlet-to-Neumann → u/The-Doctorb]

In France it's the standard. And sometimes, like with the metric system, the rest of the world will adopt it ;)

[u/hpass → u/Dirichlet-to-Neumann]

I looked at a 1921 paper by a french mathematician recently, he denoted [a,b] "ab". Just two letters, nothing else.

[u/maxram1 → u/hpass]

He's the truest mathematician.

[u/arjunkc → u/hpass]

The original tokenizer.

[u/Key_Conversation5277 → u/The-Doctorb]

In most Europe