Juji-gatame (十字固), the cross armlock.

Kansetsu-waza (joint locks).
Required for: 11th Mon (green belt, British Judo Association); 12th Mon (green belt, British Judo Association); 3rd Kyu (green belt, JudoScotland); 1st Dan (black belt, British Judo Association); 2nd Dan (black belt, British Judo Association); 3rd Dan (black belt, British Judo Association); 4th Dan (black belt, British Judo Association); 5th Dan (black belt, British Judo Association).
Also known as: armbar (bjj) - BJJ's armbar is a family of positions; juji-gatame is its classical cross-body form..

About
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The cross armlock: uke's extended arm is levered across tori's hips, the whole body bending the elbow against its hinge. The most successful armlock in competition judo. Kansetsu-waza are restricted by age in most federations; learn and practise this only under instruction.

Step by step (standard right-handed form)
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1. Your partner lies on their back; you are beside their right shoulder, controlling their right arm with both of yours, hugging it to your chest.
2. Place your left leg over their head and your right leg over their chest, then sit down close, your hips tight against their right shoulder: there must be no space between your hips and their arm.
3. Lie back, keeping the arm hugged to your chest with their thumb turned towards the ceiling, your knees pinched together over the arm.
4. Keep both of your legs heavy: the leg over their head stops them sitting up, the leg over their chest stops them rolling towards you.
5. Apply the lock by lifting your hips gently upward against the back of their elbow while the arm stays hugged straight. The pressure builds gradually; there is never a jerk.
6. The moment they tap, lower your hips and release the arm completely. In practice, apply only under instruction and within your federation's age rules.

Key points
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1. Sit close: your hips against their shoulder, no space under the arm
2. Knees pinched together, their thumb pointing to the ceiling
3. Lift your hips gently to apply; release the instant they tap

Receiving it
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Your arm is hugged away and their legs arrive over your head and chest; as their hips lift you feel a straightening pressure at the back of your elbow that builds rather than strikes. Tap early and clearly, on them or the mat, at the FIRST feeling of pressure on the joint; an elbow gives little warning between discomfort and damage, and there is nothing to be gained by waiting. Never snatch the arm out with a twist; if your partner releases, come out calmly.

VI judo notes
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Every checkpoint here is a contact, which is why armlocks travel so well into VI judo. Hips tight to their shoulder with no gap, both knees pinched over the arm, their thumb turned up in your hands. Lift the hips slowly and keep your head near the trapped arm so you can hear the tap as well as feel it, then lower and release the arm completely.

Reference pages elsewhere
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Juji-gatame on the IJF technique reference (International Judo Federation): https://judo.ijf.org/techniques/Juji-gatame

From Waza Book: https://wazabook.com/techniques/juji-gatame/