Xiangdong Ji presents a gauge-invariant decomposition of the nucleon spin into quark helicity, quark orbital, and gluon contributions. This decomposition allows for the measurement of the total quark contribution through virtual Compton scattering in a specific kinematic region where single quark scattering dominates. Deeply-virtual Compton scattering (DVCS) offers a promising method to explore the quark and gluon structure of the nucleon. The nucleon spin structure reflects non-perturbative QCD physics. Recent data on polarized deep-inelastic scattering suggest that about 20±15% of the nucleon spin is carried by quark spin or helicity. The remaining spin is attributed to quark and gluon orbital angular momenta and gluon helicity. However, these operators are not gauge-invariant in interacting gauge theories, making their experimental significance doubtful. Ji shows that the QCD angular momentum operator can be decomposed into gauge-invariant quark and gluon contributions. The quark part can be separated into helicity and orbital contributions, while the gluon part cannot. The gauge-invariant quark and gluon contributions to the nucleon spin asymptotically approach a ratio of 16:3n_f, where n_f is the number of active fermion flavors. This result matches previous findings in a non-gauge-invariant formulation. The gauge-invariant angular momentum operator allows meaningful calculations and measurements of the nucleon spin fractions carried by quarks and gluons. DVCS provides access to new nucleon observables—off-forward parton distributions—which generalize ordinary parton distributions and elastic form factors. These distributions can be measured through DVCS, offering insights into the nucleon's spin structure. The QCD angular momentum operator is defined using Lorentz transformation generators. It can be separated into gauge-invariant quark and gluon contributions. The quark angular momentum includes the usual quark helicity and a gauge-invariant orbital contribution, while the gluon angular momentum is derived from the Poynting momentum density. In the light-like gauge and infinite momentum frame, the gluon angular momentum can be measured in high-energy scattering. The gauge-invariant form of the QCD angular momentum operator allows for the calculation of the nucleon spin fractions carried by quarks and gluons. The quark and gluon contributions to the nucleon spin approach a ratio of 16:3n_f as Q² increases. This result is consistent with previous findings and provides a framework for measuring the nucleon's spin structure. DVCS is a promising method to measure the nucleon's spin structure. It involves a virtual photon absorbed by a nucleon and a real photon emitted. The kinematics of DVCS ensure that single quark scattering dominates, making it suitable for measuring the nucleon's spin structure. The process provides access to off-forward parton distributions, which are essential for understanding the nucleXiangdong Ji presents a gauge-invariant decomposition of the nucleon spin into quark helicity, quark orbital, and gluon contributions. This decomposition allows for the measurement of the total quark contribution through virtual Compton scattering in a specific kinematic region where single quark scattering dominates. Deeply-virtual Compton scattering (DVCS) offers a promising method to explore the quark and gluon structure of the nucleon. The nucleon spin structure reflects non-perturbative QCD physics. Recent data on polarized deep-inelastic scattering suggest that about 20±15% of the nucleon spin is carried by quark spin or helicity. The remaining spin is attributed to quark and gluon orbital angular momenta and gluon helicity. However, these operators are not gauge-invariant in interacting gauge theories, making their experimental significance doubtful. Ji shows that the QCD angular momentum operator can be decomposed into gauge-invariant quark and gluon contributions. The quark part can be separated into helicity and orbital contributions, while the gluon part cannot. The gauge-invariant quark and gluon contributions to the nucleon spin asymptotically approach a ratio of 16:3n_f, where n_f is the number of active fermion flavors. This result matches previous findings in a non-gauge-invariant formulation. The gauge-invariant angular momentum operator allows meaningful calculations and measurements of the nucleon spin fractions carried by quarks and gluons. DVCS provides access to new nucleon observables—off-forward parton distributions—which generalize ordinary parton distributions and elastic form factors. These distributions can be measured through DVCS, offering insights into the nucleon's spin structure. The QCD angular momentum operator is defined using Lorentz transformation generators. It can be separated into gauge-invariant quark and gluon contributions. The quark angular momentum includes the usual quark helicity and a gauge-invariant orbital contribution, while the gluon angular momentum is derived from the Poynting momentum density. In the light-like gauge and infinite momentum frame, the gluon angular momentum can be measured in high-energy scattering. The gauge-invariant form of the QCD angular momentum operator allows for the calculation of the nucleon spin fractions carried by quarks and gluons. The quark and gluon contributions to the nucleon spin approach a ratio of 16:3n_f as Q² increases. This result is consistent with previous findings and provides a framework for measuring the nucleon's spin structure. DVCS is a promising method to measure the nucleon's spin structure. It involves a virtual photon absorbed by a nucleon and a real photon emitted. The kinematics of DVCS ensure that single quark scattering dominates, making it suitable for measuring the nucleon's spin structure. The process provides access to off-forward parton distributions, which are essential for understanding the nucle