Yoko-shiho-gatame (横四方固), the side four-quarter hold.

Osaekomi-waza (hold-downs).
Required for: 7th Kai (white belt with a brown stripe, JudoScotland); 8th Kai (white belt with a black stripe, JudoScotland); 9th Kai (red belt with a white stripe, JudoScotland); 4th Mon (yellow belt, British Judo Association); 8th Mon (orange belt, British Judo Association); 5th Kyu (yellow belt, JudoScotland); 4th Kyu (orange belt, JudoScotland); 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: side control (bjj).

About
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Tori controls uke from the side at right angles, chest on chest, with grips at the collar and between the legs.

Step by step (standard right-handed form)
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1. Your partner lies on their back; take position at their right side, your body at right angles to theirs, chest facing down onto their chest.
2. Slide your right arm under the back of their neck and grip the cloth at their far shoulder or collar.
3. Reach your left arm between their legs and grip the back of their belt or the cloth at their far thigh.
4. Bring your chest down onto theirs and keep your hips low to the mat, or knees tucked in tight against their side: leave no space beneath you.
5. When they bridge, let your hips follow the lift and settle again; when they turn, slide around them like a weight on a table, chest never leaving chest.

Key points
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1. Grip under the neck and under the far leg
2. Keep hips low or knees in tight: no space under you
3. Follow uke's bridge with your hips, chest stays heavy

Receiving it
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Chest lands on chest from your side, at right angles, with grips at your collar and between your legs: the pressure is broad and central, and bridging lifts them but does not shift them unless you follow with your hips. Turn towards them as you bridge, hunting an elbow or a knee inside. Settle your breathing between attempts rather than spending everything at once; tap clearly if you need to stop.

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

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