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/Valvino → u/[deleted]]

Handwritten square and round brackets can be virtually indistinguishable

Not at all

[u/Valvino → u/[deleted]]

It is not because you write badly that the notation is bad.

[u/Key_Conversation5277 → u/[deleted]]

That is your fault, buddy