There are two common conventions for private fields: camelCase and _camelCaseWithLeadingUnderscore.

Camel case

public class Rational
{
    private readonly int numerator;
    private readonly int denominator;

    public Rational(int numerator, int denominator)
    {
        // "this" keyword is required to refer to the class-scope field
        this.numerator = numerator;
        this.denominator = denominator;
    }
}

Camel case with underscore

public class Rational
{
    private readonly int _numerator;
    private readonly int _denominator;

    public Rational(int numerator, int denominator)
    {
        // Names are unique, so "this" keyword is not required
        _numerator = numerator;
        _denominator = denominator;
    }
}