Em/C# is C#, E, G and B. The shape this page opens on is written x42000, lowest string first.

Also written Em over C# or Em with C# in the bass. The same chord either way; which spelling you meet depends on who wrote the chart.
There are 3 common shapes for Em/C# on the guitar.
In which keys. Every note of Em/C# belongs to D major (the ii) and B minor (the iv).
The common shapes, written lowest string first: x42000, x45457, 9-10-9-12-12-12.
The common shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.