The most important feature of urticaria is that the swellings come and go. Weals are nearly always itchy. They usually fade within a day or less before coming up somewhere else. Angioedema swellings often last longer and may be uncomfortable. Individual swellings may happen spontaneously or only come up with certain specific physical triggers, such as mechanical pressure or temperature change. This is why urticaria is now classified as being spontaneous or inducible. Spontaneous weals generally fade within 24 hours whereas induced weals go within an hour, with the exception of delayed pressure urticaria, so the duration of individual weals can be a useful diagnostic pointer in the history. An individual patient may have both types together. Other conditions that behave a bit like urticaria but have a different diagnosis need to be recognized because they will need different investigation and treatment. They include urticarial vasculitis, angioedema without weals and some very rare syndromes with urticaria as a component.