Because they are generally found in pairs, they are often referred to by their plural form: venae comitantes.

Venae comitantes are usually found with certain smaller arteries, especially those in the extremities.

Larger arteries, on the other hand, generally do not have venae comitantes.

They usually have a single, similarly sized vein which is not as intimately associated with the artery.

Claude Bernard suggested the interchange of heat between the arteries and adjacent veins might moderate cooling of the arterial blood, for which there is experimental evidence.