emphasizing the ability to relax the mind and body even under the stress of dangerous situations. This is necessary to enable the practitioner to perform the 'enter-and-blend' movements that underlie aikido techniques, wherein an attack is met with confidence and directness. Morihei Ueshiba once remarked that one "must be willing to receive 99% of an opponent's attack and stare death in the face" in order to execute techniques without hesitation. As a martial art concerned not only with fighting proficiency but with the betterment of daily life, this mental aspect is of key importance to aikido practitioners. Uniforms and ranking Aikido practitioners (commonly called outside Japan) generally progress by promotion through a series of "grades" (), followed by a series of "degrees" (), pursuant to formal testing procedures. Some aikido organizations use belts to distinguish practitioners' grades, often simply white and black belts to distinguish and grades, although some use various belt colors. Testing requirements vary, so a particular rank in one organization is not comparable or interchangeable with the rank of another. Some dōjōs have an age requirement before students can take the rank exam. The uniform worn for practicing aikido () is similar to the training uniform () used in most other modern martial arts; simple trousers and a wraparound jacket, usually white. Both thick ("judo-style"), and thin ("karate-style") cotton tops are used. Aikido-specific tops are available with shorter sleeves which reach to just below the elbow. Most aikido systems add a pair of wide pleated black or indigo trousers known as (used also in Naginatajutsu, kendo, and iaido). In many schools, the wearing of is reserved for practitioners with ranks or for instructors, while others allow all practitioners to wear a regardless of rank. Aikido styles Aikido styles vary in their intention as due to its holistic nature. The most common differences noted in aikido, when observed externally, relate to the intensity and realism of training. Stanley Pranin has observed that some criticism may stem from weak attacks from , allowing for a conditioned response from , resulting in underdevelopment of the skills needed for the safe and effective practice of both partners. To counteract this, some styles allow students to become less compliant over time, but, in keeping with the core philosophies, this is after having demonstrated proficiency in being able to protect themselves and their training partners. Shodokan Aikido addresses the issue by practicing in