Use setopt for user options (defcustom)

Introduced in Emacs 29. Preferred for all defcustom variables in your init file.

(setopt display-line-numbers-type 'relative)
(setopt copilot-idle-delay 0.5)

Why: setopt runs the :set callback defined by defcustom and validates the value against its :type. Using setq on a defcustom variable bypasses both, which can silently break packages that depend on the setter to update internal state.

Caveat: slower than setq because it goes through the Customize machinery. Don't use it for regular variables in hot paths.

Works in early-init.el: setopt is autoloaded (via loaddefs.el, which is part of the Emacs dump), so it is available during early-init.el. The first use triggers loading cus-edit.el slightly earlier than normal — negligible overhead for a few variables.

Use setq for regular variables

The standard way to set any variable. Use for internal/non-customizable variables.

(setq gc-cons-threshold (* 50 1000 1000))
(setq x 1
      y 2)

Note: setq sets the current binding (lexical or dynamic). Inside a let, it modifies the local binding, not the global one.

Use setq-default for buffer-local defaults

Sets the default value for buffer-local variables. The value applies to buffers that haven't set their own local value.

(setq-default indent-tabs-mode nil)
(setq-default fill-column 80)

Use setq-local for buffer-local values

Sets a variable's value in the current buffer only.

(add-hook 'org-mode-hook
  (lambda () (setq-local fill-column 100)))

Use set when the variable name is computed

A function (not a special form) -- evaluates its first argument to get the symbol to set.

(set 'my-var 42)          ; equivalent to (setq my-var 42)
(set (intern name) value)  ; dynamic symbol lookup

Rarely needed in init files.

Quick decision table

Sources

• GNU Elisp Manual -- Setting Variables
• Emacs Redux -- Goodbye setq, Hello setopt