Quadrangular space: axillary nerve.

Remember: a is before r in quadrangular.

Triangular space: radial nerve.

Remember: r is before a in triangular.

References

1. Hacking, C. and Gaillard, F. (2017). Quadrangular space | Radiology Reference Article | Radiopaedia.org. [online] Radiopaedia. Available at: https://radiopaedia.org/articles/quadrangular-space [Accessed 25 Aug. 2017].
2. Knipe, H. (2017). Lateral triangular space | Radiology Reference Article | Radiopaedia.org. [online] Radiopaedia. Available at: https://radiopaedia.org/articles/lateral-triangular-space [Accessed 25 Aug. 2017].
3. Duke Medicine. (2017). Duke Anatomy – Lab 10: Shoulder, Axilla, & Arm. [online] Available at: https://web.duke.edu/anatomy/lab10/lab10.html [Accessed 25 Aug. 2017].