p.212
The bayḍah
    This "egg-like" helmet was probably round rather than pointed. It may, of course, have had an oval or domed shape but is unlikely to have been of a segmented spangenhelm, construction. The latter system generally produced a more pointed outline than the two-piece helmet of late-Roman or Persian "Parthian Cap" type. These latter two fashions (Figs. 9, 40, 56, 85, 95 and 618) are likely to have corresponded to the original Arab bayḍah though there is little reason to suppose that this term was not used more broadly in later centuries. A surviving helmet of very uncertain date from Tunisia could show that this form of construction, in which the constituent parts were joined fore-and-aft, persisted well into the Middle Ages (Fig. 191). An extension to protect the wearer's neck on this particular North African helmet might, perhaps, be the dabirah or rear of a bayḍah helmet which itself could be fastened to a hauberk.⁸ A very similar neck extension is illustrated in an astrological manuscript from Morocco, dated 1224 AD (Book of fixed Stars, by al Sūfī, Vatican Lib., Ms. Ross 1033, f. 6), although here it is fastened to a spangenhelm. Nor would references to the bayḍah being festooned with rivets⁹ necessarily contradict the above interpretation.

8. Ibn Hudhayl, op. cit., pp. 264-268; Schwarzlose, op. cit., p. 349.
9. Schwarzlose, op. cit., p. 351.

p.213
Rivets were, of course, used in large numbers to construct many "Parthian Cap" helmets, though less so in later-Roman styles.
    A wider use of the name bayḍah or perhaps a later fashion of adding decorative spikes or plumes¹⁰ could account for the fact that in the early 14th century a warrior was advised to remove his bayḍah helmet before trying to take off or put on a hauberk.¹¹
    The bayḍah was also large enough to be worn over a michfar or coif.¹² It could be held in place by straps or laces,¹³ though whether these went under the chin or were drawn around the temples and tied at the back is not clear. Both systems were known in Byzantium and western Europe,¹⁴ but the latter seems to have been more popular in the Middle East, perhaps being comfortable in hot weather (Figs. 119, 216, 330, 331, 337, 340, 348 and 445).
    Round helmets, with or without fixed or removeable neck-guards, brims or other extra features, are very common in the 7th to 13th century art of the area under study, even if one excludes those of apparent segmented construction (Figs. 13, 132, 134, 178B, 190A, 196, 201, 207, 211, 220, 235, 251, 256, 291, 292, 300, 316, 331, 337, 338, 339, 367, 378, 392, 401, 410, 419, 422, 447, 494, 515, 520, 524, 525, 526, 545, 580, 597, 604M and 610).

10. Ibn Hudhayl, loc cit.
11. Al Aqsarā'ī, op. cit., p. 319.
12. Al Ṭabarī, op. cit., vol. II, p. 1278; Canard, Sayf al Daule, Recueil de textes, p. 212.
13. Beshir, op. cit., pp. 67-70; al Agsarā'ī, op. cit., p. 320; Anon., The Song of Roland, verses 55 and 79.
14. Haldon, "Some Aspects of Byzantine Military Technology from the 6th to the 10th centuries," p. 21.