How add condition in jquery validation

Question

I have some validation in form. I would like to add some other validation. in firstname textfield allow only alphabet text.

what keyword to be add in this jquery for alphabets only.

Here is the code that does the validating for form.

$("#validateForm").validate({
    debug: false,
    rules: {
        firstname: {
            required: true,
            maxlength: 10
        },
        lastname: {
            required: true,
            maxlength: 10
        },
        contactmail: {
            required: true,
            email: true
        },
        jobtitle: "required",
        username: {
            required: true,
            minlength: 2
        },
        password: {
            required: true,
            minlength: 5
        },
        repassword: {
            required: true,
            minlength: 5,
            equalTo: "#password"
        }

    },
    messages: {
        firstname: {
            required: "Please enter your firstname",
            maxlength: "firstname must consist of below 10 characters"
        },
        lastname: {
            required: "Please enter your lastname",
            maxlength: "lastname must consist of below 10 characters"
        },
        contactmail: {
            required: "Please enter email address",
            email: "Email must be in the format of name@domain.com"
        },
        jobtitle: "Please enter job title",
        username: {
            required: "Please enter a username",
            minlength: "Your username must consist of at least 2 characters"
        },
        password: {
            required: "Please provide a password",
            minlength: "Your password must be at least 5 characters long"
        },
        repassword: {
            required: "Please provide a retype password",
            minlength: "Your password must be at least 5 characters long",
            equalTo: "Please enter the same password as above"
        }

    },
});