Edit: this is from the Apple naming guidelines, but it doesn't clarify that they always follow that convention for private methods: Avoid the use of the underscore character as a prefix meaning private in method names (using an underscore character as a prefix for an instance variable name is allowed). Apple reserves the use of this convention. Use by third parties could result in name-space collisions; they might unwittingly override an existing private method with one of their own, with disastrous consequences. See “Private Methods” for suggestions on conventions to follow for private API.