Variant Letters
In catering to the fine points of mathematical typesetting and notational clarity, the Wolfram Language provides a variety of special variant forms of letters.
Wolfram Language Notation and Doubled Letters
\[ImaginaryI] — special form of 
representing 
(entered as 
ii
)
\[ConstantC] ▪ \[DifferentialD] ▪ \[ExponentialE] ▪ \[ImaginaryJ]
\[DoubledGamma] ▪ \[DoubledPi]
Special Letter-Based Forms
µ \[Micro] ▪ Å \[Angstrom] ▪ ℏ \[HBar] ▪ ℘ \[WeierstrassP]
Variant Greek Letters
ε \[CurlyEpsilon] ▪ ϑ \[CurlyTheta] ▪ ϰ \[CurlyKappa] ▪ ϖ \[CurlyPi] ▪ ϱ \[CurlyRho] ▪ ς \[FinalSigma] ▪ φ \[CurlyPhi] ▪ ϒ \[CurlyCapitalUpsilon]
Dotless Letters
ı \[DotlessI] — dotless i, used for unit vectors with hats, etc.