Abstract: Compound fenestration assembly mull joints using joining strips and compound fenestration assemblies using those mull joints are described herein. The joining strips used in the compound fenestration assembly mull joints described herein are configured to improve the strength of the mull joints in which they are used and, therefore, the ability of the resulting compound fenestration assemblies to maintain integrity when subjected to, e.g., wind loads and other forces is improved. The benefits of the mull joints and the compound fenestration assemblies incorporating them as described herein may be enhanced with an increase in the number of adjacent fenestration units (and, therefore, mull joints) in a given compound fenestration assembly.