Margins called by Clearing Houses whenever a new position is opened are often deemed to be good indicators of the riskiness of the position. This approach complies with IOSCO recommendation to maintain sufficient financial resources to withstand, at a minimum, a default by the participant to which it has the largest exposure in extreme but plausible market conditions. This statement clearly indicates that initial margins should account for the position's riskiness in a way similar to the Value-at-Risk of the overall position. In order to achieve this assessment, each Clearing House adopts rules of its own, but all rely on quite a few basic approaches, known as TIMS, SPAN and RIVA. These algorithms basically differ in how they account for volatilities and offsetting positions. The goal of the paper is to highlight such differences, by computing the margins required by each method for complex positions, designed for the purpose, by simulating their behaviour over time. In other words, we perform a kind of stress-test, to examine how margins react to "extreme" situations. As a result, it will be apparent that some algorithms overestimate riskiness, and therefore require initial margins higher than due

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