The child resistant safety closure is one that uses a common single start thread on the neck of a container and on a closure. In conjunction with this single start thread there is a single mating projection on the neck and on the closure. These mating projections are in an interference fit until the closure is distorted so that the closure bows outwardly in the region of the projections whereby the projection on the neck and the projection on the closure can pass by one another. The distortion is enhanced by having the plastic of the closure in the area of the projection of a decreased thickness and the neck having flattened portions to provide for a easy distortion of the closure. By the use of a single start common thread and one projection the same closure can be used for containers requiring a safety closure and those not requiring a safety closure. The difference will solely be in the neck finish of the container or on the inner surface of the closure. When the container is to have a safety closure the neck finish has a projection to mate with a projection on the closure. In the instance where the container is not to have a child resistant safety closure either the closure will not have a projection or the neck of the container will not have a projection. In such a case the closure can be freely removed from the container.